mwl8k: bump the transmit wait timeout to 5 seconds

While it is reasonable to expect that at least one transmit ring entry
will be processed per second while we are waiting for the transmit
rings to drain, the firmware can end up doing batching of transmit ring
status writeback, which means that the transmit rings can appear stuck
for more than a second at a time.

Bump the TX drain wait timeout up from 1 to 5 seconds to account for
this.

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