wifi: cfg80211: expose nl80211_chan_width_to_mhz for wide sharing

The newly added WBRF feature needs this interface for channel
width calculation.

Signed-off-by: Evan Quan <quanliangl@hotmail.com>
Signed-off-by: Ma Jun <Jun.Ma2@amd.com>
Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Link: https://msgid.link/20231211100630.2170152-4-Jun.Ma2@amd.com


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