wil6210: allow sending special packets when tx ring is full

When traffic is very high, some special packets such as ARP may get
dropped causing the tx traffic to stall intermittently.
This patch preserves spare tx descriptors so that special packets can
be sent even when the tx ring is almost full. The number of reserved
entries can be changed by writing to tx_reserved_entries via debugfs,
taking effect in subsequent connections.

This feature is enabled when either drop_if_ring_full or ac_queues are
enabled.

Change-Id: I6c5215500966840445f2631fb32494750e60303d
Signed-off-by: Hrishikesh Vidwans <hvidwans@codeaurora.org>
Signed-off-by: Ahmad Masri <amasri@codeaurora.org>