brcmfmac: fix regression in brcmf_sdio_txpkt_hdalign()

Recent change in brcmf_sdio_txpkt_hdalign() changed the
behavior and now always returns 0. This resulted in a
regression which basically renders the device useless.

Fixes: 270a6c1f

 ("brcmfmac: rework headroom check in .start_xmit()")
Reported-by: S. Gilles <sgilles@math.umd.edu>
Tested-by: S. Gilles <sgilles@math.umd.edu>
Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>