Staging: brcmfmac: Fix MAC header lookup on 64-bit architectures

Fix direct use of sk_buff::mac_header which is an offset rather
than a pointer on 64-bit architectures.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Cc: Brett Rudley <brudley@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>