Update gps.c

correctly verifies checksum before modifying read NMEA gps data
taking a stab at returning "permanent" IDs from MSP_BOXIDS. Let's see if
this fixes stuff nobody uses.
Cleanup GPS compilation warnings from
ca746d2ff47c8eb589386045b91bbccdd76246d1
CC3D / NAZE - Adding support for hardware controlled inverter for use by
SBUS RX.
Fix PWM in RSSI - accidentally excluded when adding CJMCU support.
allow controlling external XOR gate connected to PB2 (BOOT1) to invert USART2_RX signal on F1. Currently used by sbus driver.
On second thought, no need to make inverter configurable. only sbus will use it, and the driver can set inversion automatically.