brcmfmac: support removing AP interfaces with "interface_remove"

New firmwares (e.g. 10.10.69.36 for BCM4366) support "interface_remove"
for removing interfaces. Try to use this method on cfg80211 request. In
case of older firmwares (e.g. 7.35.177.56 for BCM43602 as I tested) this
will just result in firmware rejecting command and this won't change any
behavior.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>