staging: brcm80211: remove sparse warnings from mac80211_if.c

Some changes have been made to get rid of sparse warnings in the
source file mac80211_if.c.

Reported-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Reviewed-by: Roland Vossen <rvossen@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Roland Vossen <rvossen@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>