mac80211: Fix calculation of minimal channel width

When calculating the minimal channel width for channel context,
the current operation Rx channel width of a station was used and not
the overall channel width capability of the station, i.e., both for
Tx and Rx.

Fix ieee80211_get_sta_bw() to use the maximal channel width the
station is capable. While at it make the function static.

Signed-off-by: Ilan Peer <ilan.peer@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20201206145305.4387040b99a0.I74bcf19238f75a5960c4098b10e355123d933281@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>