Use correct EC command to fetch keyboard backlight level

[kelnos]

The EC has a dedicated command for fetching the backlight level; use that instead of checking PWM duty cycle, which doesn't appear to work (catting the 'brightness' sysfs node always returns 0, regardless of the actual backlight level).

This is also what framework_tool does when passed '--kblight' with no options.

[DHowett]

Thanks for the patch! An earlier version of this module did use EC_CMD_PWM_GET_KEYBOARD_BACKLIGHT -- however, on the Ryzen 7040 Framework Laptop 13 it reports that the backlight is disabled. It's fair enough to say that we should ignore that field. 🙂

[kelnos]

Hm, I guess we could also detect the chipset/platform and act accordingly?

[kelnos]

Also, does the duty cycle method work on the AMD board? And does the GET_KEYBOARD_BACKLIGHT command work? Just the enabled field is broken on the AMD board?