mwl8k: allow more time for transmit rings to drain

Before issuing any firmware commands, we wait for the transmit rings
to drain, to prevent control versus data path synchronization issues.
In some cases, this can end up taking longer than the current hardcoded
limit of 5 seconds, for example if the transmit rings are filled with
packets for a host that has dropped off the air and we end up
retransmitting every pending packet at the lowest rate a couple of
times.

This patch changes mwl8k_tx_wait_empty() to only bail out on timeout
expiry if there was no change in the number of packets pending in the
transmit rings during the waiting period.  If at least one transmit
ring entry was reclaimed while we were waiting, we are apparently still
making progress, and we'll allow waiting for another timeout period.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>