wl1251: update tx_hdr when aliging skb in tx

Before transmission when aligning the buffer to 4-byte bounday, tx_hdr
needs to be updated. Otherwise debug logs print false data.

Signed-off-by: Kalle Valo <kalle.valo@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>