wil6210: add option to drop Tx packets when Tx ring is full

In AP mode with multiple clients, driver stops net queue
(netif_tx_stop_queue) upon first ring (serving specific client)
becoming full. This can have negative effect on transmission to
other clients which may still have room in their corresponding rings.

Implement new policy in which stop/wake net queue are not used. In
case there is no room in the ring for a transmitted packet, drop the
packet.

New policy can be helpful to debug performance issues, to guarantee
maximum utilization of net queues.
New policy is disabled by default and can be enabled by debugfs:
echo 1 > drop_if_ring_full

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