mwifiex: increase TX threashold to avoid TX timeout during ED MAC test

While carrying energy detection(ED) tests, the chip will stop
transmission upon detecting an energy in the connected channel.
As a feedback, driver will stop dequeuing TX packets and due to
which wmm_tx_pending keep incremeting. Once wmm_tx_pending
reaches 100, driver calls netif_tx_stop_queue(). If TX ques is
not restarted within 5(watchdog_timeo) seconds, it will result in
TX timeout.

The ED test is carried out for one minute and the current
threshold of 100 is easily overcome by the traffic, cuasing TX
timeouts. To fix this increase the threshold to 400.

Signed-off-by: Cathy Luo <cluo@marvell.com>
Signed-off-by: Ganapathi Bhat <gbhat@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>