brcmfmac: use lock in brcmf_fws_del_interface()

When deleting an interface in firmware-signalling module it will
clear any destination descriptors. To avoid concurrency issues it
should take the lock using brcmf_fws_lock().

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