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.

The vring debugfs entries have changed as follows:
- dbg_vring_index has changed to dbg_ring_index
- vrings has changed to rings
- vring_idle_trsh has changed to ring_idle_trsh
- vring_index has changed to ring_index

Signed-off-by: Gidon Studinski <gidons@codeaurora.org>
Signed-off-by: Maya Erez <merez@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>