[peripherals] mpu60x0: use gcc pragma to ignore cast-qual warnings

The buffer is volatile, since filled from ISR...
But we know it's ok to use it here so we silence the warning.

Also there is a bug in some gcc version that incorrectly reports this as
peripherals/mpu60x0_spi.c:118:33: warning: cast discards '__attribute__((noreturn))' qualifier from pointer target type [-Wcast-qual]
see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55383
[peripherals] ignore cast-qual for mpu9250 as well
Merge pull request #946 from paparazzi/mpu60x0_ignore_cast_warning

[peripherals] mpuxxxx: use gcc pragma to ignore cast-qual warnings

The buffer is volatile, since filled from ISR...
But we know it's ok to use it here so we silence the warning.

Also there is a bug in some gcc version that incorrectly reports this as
peripherals/mpu60x0_spi.c:118:33: warning: cast discards '__attribute__((noreturn))' qualifier from pointer target type [-Wcast-qual]
see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55383