AP_AHRS: make a pair of methods static on AP_AHRS_Backend

[peterbarker]

these don't use any information from the specific backend, but they are called within the backends so making them static on that class is handy

[peterbarker]

Board               AP_Periph  blimp  bootloader  copter  heli  iofirmware  plane  rover  sub
Durandal                       -24    *           -24     -24               -24    -24    -24
HerePro             *                                                                     
Hitec-Airspeed      *                                                                     
KakuteH7-bdshot                -16    *           -16     -16               -16    -16    -16
MatekF405                      -16    *           -16     -16               -16    -16    -16
Pixhawk1-1M-bdshot             -24                -24     -24               -16    -24    -24
f103-QiotekPeriph   *                                                                     
f303-Universal      *                                                                     
iomcu                                                           *                         
revo-mini                      -16    *           -16     -16               -16    -16    -16
skyviper-v2450                                    -16