staging: brcm80211: replace occurrences of __attribute__((packed))

One of the checkpatch warning on this driver is about usage of the
__attribute__ keyword. This patch changes this to using the __packed
keyword.

Reviewed-by: Roland Vossen <rvossen@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>