brcmfmac: fix incorrect event channel deduction

brcmf_sdio_fromevntchan() was being called on the the data frame
rather than the software header, causing some frames to be
mischaracterized as on the event channel rather than the data channel.

This fixes a major performance regression (due to dropped packets). With
this patch the download speed jumped from 1Mbit/s back up to 40MBit/s due
to the sheer amount of packets being incorrectly processed.

Fixes: c56caa9d ("brcmfmac: screening firmware event packet")
Signed-off-by: Gavin Li <git@thegavinli.com>
Cc: <stable@vger.kernel.org> # 4.7+
Acked-by: Arend van Spriel <arend.vanspriel@broadcom.com>
[kvalo@codeaurora.org: improve commit logs based on email discussion]
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>