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.

Change-Id: I3ffc9456e8245865ba4e883aec1d2a03c0c9eb2e
Signed-off-by: Dedy Lansky <dlansky@codeaurora.org>