wl1271: Fix RX data path frame lengths

The current frame length used by the driver for RX frames is the SPI bus
transfer length. This length has padding bytes, which do not belong to the
WLAN frame.

As there is no other length information in the WLAN frame except the skb
length this problem caused for instance extra ESSID's to be listed at the
end of scan results (IE id 0) with zero length.

Fix the frame length by removing padding.

Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Reviewed-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>