mwifiex: define generic data type for PCIe ring buffers

This patch defines PCIe ring buffer array pointer as void instead
of mwifiex_pcie_buf_desc. This will enable us to use same pointers
for ring operations instead of new structures if buffer descriptor
structure changes.

Also split out event buffer descriptor structure from struct
mwifiex_pcie_buf_desc. For PCIe8766 TX/RX buffer descriptor is
same as evevt buffer descriptor. Newer chips could use different
TX/RX buffer descriptor while event descriptor remains the same.

Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>