brcmfmac: properly align buffers on certain platforms with 64 bit DMA

Systems with 64 bit DMA at least partially require buffers to be used
for DMA to be 8-byte-aligned. One example is Amlogic Meson GX.
Switching the MMC/SDIO driver for this platform to SG DMA mode
resulted in problems due to unaligned buffers.

Fortunately the brcmfmac driver has a global define for the alignment.
Changing it to 8 fixed the issues with Meson GX.

Suggested-by: Helmut Klein <hgkr.klein@gmail.com>
Tested-by: Helmut Klein <hgkr.klein@gmail.com>
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>