staging: brcm80211: replace 0 integer with NULL pointer

Running sparse showed following warning: Using plain integer as NULL
pointer. This has been fixed.

Reported-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Reviewed-by: Franky (Zhenhui) Lin <frankyl@broadcom.com>
Reviewed-by: Roland Vossen <rvossen@broadcom.com>
Signed-off-by: Roland Vossen <rvossen@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>