mac80211: twt: don't use potentially unaligned pointer

Since we're pointing into a frame, the pointer to the
twt_agrt->req_type struct member is potentially not
aligned properly. Open-code le16p_replace_bits() to
avoid passing an unaligned pointer.

Reported-by: kernel test robot <lkp@intel.com>
Fixes: f5a4c24e ("mac80211: introduce individual TWT support in AP mode")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Link: https://lore.kernel.org/r/20210927115124.e1208694f37b.Ie3de9bcc5dde5a79e3ac81f3185beafe4d214e57@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>