mac80211: use common code for monitor options in add/change

mac80211: use common code for monitor options in add/change

Refactor the code to have common code for changing monitor
options when adding and changing virtual interfaces. This
will make it easier to add BPF filters to both paths. Note
that this code carefully checks the error conditions first
and only then applies the changes, to guarantee atomicity.

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