wil6210: add support for enhanced DMA debugfs

Add debugfs support for enhanced DMA TX and RX descriptor rings,
TX and RX status rings and RX buffer management.

Run the following command to print the TX and RX status rings:
cat srings

Run the following command in order to select the status ring:
echo STATUS_RING_IDX > dbg_sring_index

Run the following command in order to select the status message:
echo STATUS_MSG_IDX > dbg_status_msg_index

Run the following command in order to print the selected status
message from the selected status ring:
cat status_msg

Run the following command in order to print the RX buffer management
debug information:
cat rx_buff_mgmt
Signed-off-by: Gidon Studinski <gidons@codeaurora.org>
Signed-off-by: Maya Erez <merez@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>