wifi: mt76: mt7915: fix possible unaligned access in mt7915_mac_add_twt_setup

Fix possible unaligned pointer in mt7915_mac_add_twt_setup routine.

Reported-by: kernel test robot <lkp@intel.com>
Fixes: 3782b69d

 ("mt76: mt7915: introduce mt7915_mac_add_twt_setup routine")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>