mwifiex: fix tx_info/rx_info overlap with PCIe dma_mapping

On PCIe Tx data path, network interface specific tx_info
parameters such as bss_num and bss_type are saved at
"skb->cb + sizeof(dma_addr_t)" (returned by MWIFIEX_SKB_TXCB).
Later mwifiex_map_pci_memory() called from
mwifiex_pcie_send_data() will memcpy
sizeof(struct mwifiex_dma_mapping) bytes to save PCIe DMA
address and length information at beginning of skb->cb.
This accidently overwrites bss_num and bss_type saved in skb->cb
previously because bss_num/bss_type and mwifiex_dma_mapping data
overlap.
Similarly, on PCIe Rx data path, rx_info parameters overlaps
with PCIe DMA address and length information too.

Fix it by defining mwifiex_cb structure and having
MWIFIEX_SKB_TXCB and MWIFIEX_SKB_RXCB return the correct address
of tx_info/rx_info using the structure members.

Also add a BUILD_BUG_ON to maks sure that mwifiex_cb structure
doesn't exceed the size of skb->cb.

Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Signed-off-by: Chin-Ran Lo <crlo@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>