mwifiex: adjust high/low water marks for tx_pending queue

This is to fix an issue that the throughput of the higher priority
stream gets dropped when a lower priority stream is present.

Signed-off-by: Marc Yang <yangyang@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>