wil6210: [DEBUG] Improve Vring printing

Print '_' for the 'idle' descriptors - this makes vring representation more visible.
Also, for the Tx side, differentiate descriptors having associated skb's - print ones
with skb as 'H' and without as 'h'. Good to represent scattered frames.

Signed-off-by: Vladimir Kondratiev <qca_vkondrat@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>