Tidy use of backend type in AP_GPS

[peterbarker]

Also uses get_type in a few spots to avoid external callers doing something nasty.

Board                    AP_Periph  blimp  bootloader  copter  heli  iofirmware  plane  rover  sub
CubeOrange-periph-heavy  -32                                                     
Durandal                            48     *           56      48                56     56     48
Hitec-Airspeed           *                                                       
KakuteH7-bdshot                     80     *           80      80                80     80     80
MatekF405                           64     *           64      72                64     64     72
Pixhawk1-1M-bdshot                  64                 56      64                56     64     64
f103-QiotekPeriph        *                                                       
f303-Universal           64                                                      
iomcu                                                                *           
revo-mini                           64     *           72      72                64     72     72
skyviper-v2450                                         64                        

In preparation for making AP_GPS_Params

[peterbarker]

I've tested this on my lots-of-GPS vehicle, and it still finds both GPSs and they still act appropriately.