mac80211: use bitfield helpers for BA session action frames

Use the appropriate bitfield helpers for encoding and decoding
the capability field in the BA session action frames instead of
open-coding the shifts/masks.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20201206145305.0c46e5097cc0.I06e75706770c40b9ba1cabd1f8a78ab7a05c5b73@changeid


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