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 is disabled by default and can be enabled with new
drop_if_ring_full module param.

Change-Id: I862545b7f4930bb42e37ec36d0558eb8a5c3cefe
Signed-off-by: Dedy Lansky <dlansky@codeaurora.org>
Signed-off-by: Ahmad Masri <amasri@codeaurora.org>