brcmfmac: fix unaligned access in TXSTATUS signal handling

reported by Hante. Needs to be squashed in commit 187fbcec.

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