Fix issue for all LTC681x where PEC error and results from selftests are never reported in some cases.

[gudnimg]

This issue addresses #55

Below is a detailed description of the issues that are fixed.

Function	Chip	Expected Behaviour	Actual Behaviour
LTC6810_rdaux()	LTC6810	Returns 0 if no errors detected. Returns -1 if a PEC mismatch is detected.	Always returns 0. Which means Errors are not detected.
LTC6810_rdcomm()	LTC6810	Returns 0 if no errors detected. Returns -1 if a PEC mismatch is detected.	Always returns 0. Which means Errors are not detected.
LTC6810_run_adc_redundancy_st()	LTC6810	Return the total number of errors detected.	Always returns 0. Which means Errors are not detected.
LTC6811_rdaux()	LTC6811	Returns 0 if no errors detected. Returns -1 if a PEC mismatch is detected.	Always returns 0. Which means Errors are not detected.
LTC6811_rdcomm()	LTC6811	Returns 0 if no errors detected. Returns -1 if a PEC mismatch is detected.	Always returns 0. Which means Errors are not detected.
LTC6811_run_adc_redundancy_st()	LTC6811	Return the total number of errors detected.	Always returns 0. Which means Errors are not detected.
LTC6811_run_adc_overlap()	LTC6811	Return the total number of errors detected.	Always returns 0. Which means Errors are not detected.
LTC6812_rdsctrl()	LTC6812	Returns 0 if no errors detected. Returns -1 if a PEC mismatch is detected.	returns nothing. Undefined behaviour if return value is used.
LTC6812_rdaux()	LTC6812	Returns 0 if no errors detected. Returns -1 if a PEC mismatch is detected.	Always returns 0. Which means Errors are not detected.
LTC6812_rdcomm()	LTC6812	Returns 0 if no errors detected. Returns -1 if a PEC mismatch is detected.	Always returns 0. Which means Errors are not detected.
LTC6812_run_adc_redundancy_st()	LTC6812	Return the total number of errors detected.	Always returns 0. Which means Errors are not detected.
LTC6813_rdsctrl()	LTC6813	Returns 0 if no errors detected. Returns -1 if a PEC mismatch is detected.	returns nothing. Undefined behaviour if return value is used.
LTC6813_rdaux()	LTC6813	Returns 0 if no errors detected. Returns -1 if a PEC mismatch is detected.	Always returns 0. Which means Errors are not detected.
LTC6813_rdcomm()	LTC6813	Returns 0 if no errors detected. Returns -1 if a PEC mismatch is detected.	Always returns 0. Which means Errors are not detected.
LTC6813_run_adc_redundancy_st()	LTC6813	Return the total number of errors detected.	Always returns 0. Which means Errors are not detected.