mac80211: consider virtual mon when calculating min_def

When calculating the current max bw required for
a channel context, we didn't consider the virtual
monitor interface, resulting in its channel context
being narrower than configured.

This broke monitor mode with iwlmvm, which uses the
minimal width.

Reported-by: Ido Yariv <idox.yariv@intel.com>
Signed-off-by: Eliad Peller <eliadx.peller@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>