wil6210: enhancements for descriptor and status ring debugfs

Add printing of hwhead as part of desc ring and status ring debugfs.
Validate wil->rx_buff_mgmt and ring->ctx arrays before accessing them.
Dump Tx descriptor content even when it indicates invalid buff_id.

Signed-off-by: Dedy Lansky <dlansky@codeaurora.org>
Signed-off-by: Maya Erez <merez@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>