mt76: fix endianness sparse warnings

Fix many warnings with incorrect endian assumptions.

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Reviewed-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>