brcmfmac: remove monitor interface when detaching

commit 4f61563d

 upstream.

This fixes a minor WARNING in the cfg80211:
[  130.658034] ------------[ cut here ]------------
[  130.662805] WARNING: CPU: 1 PID: 610 at net/wireless/core.c:954 wiphy_unregister+0xb4/0x198 [cfg80211]

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>