mwl8k: Remove BSSID from the firmware when the BSS is stopped

Using command DEL_MAC_ADDR, remove the mac address of the BSS
when it is stopped i.e the corresponding vif is removed. Without
this, the stale bss entry will still be maintained in the firmware
which causes issues when the BSS's are recreated.

Signed-off-by: Nishant Sarmukadam <nishants@marvell.com>
Signed-off-by: Yogesh Ashok Powar <yogeshp@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>