mwifiex: balance dma map/unmap sizes

Depending on the underlying DMA implementation its
not possible to partially unmap DMA buffers. Moreover
its not possible to understand the intent of passing
0 as the size to dma unmap. The intent of this
driver is unmap the entire skb buffer. The only way
to ensure that the size matches on unmap is to store
both the dma address and the size in the skb ca field.

Introduce a mwifiex_dma_mapping structure which tracks
the dma address and the size. Additionally, provide
a mwifiex_unmap_pci_memory() that utilizes the new
structure. This also provide symmetry within the
internal API.

Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Paul Stewart <pstew@chromium.org>
Reviewed-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>