brcmfmac: implement support for deleting virtual interfaces

Deletion of the virtual interface was not fully supported so
adding it now. Tested using p2p_connect command in wpa_supplicant.

Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>