wil6210: add support for enhanced DMA structures

In enhanced DMA the vrings are handled internally by the FW
and are not exposed to the driver.
Instead, the driver handles descriptor rings, which are mapped
by the FW to vrings.
The completions of the TX and RX descriptors are notified to
the driver using status rings. Each status ring descriptor
includes information of the completed descriptors and the ring id
of their descriptor ring.

This patch changes struct vring to generic wil_ring to allow
its reuse for enhanced DMA descriptor rings and adds the descriptor
and status rings specific descriptors.

Change-Id: I7c841e0a1a31363c86291393a6197116095842bb
Signed-off-by: Gidon Studinski <gidons@codeaurora.org>
Signed-off-by: Maya Erez <merez@codeaurora.org>