wl12xx: Increase low & high TX watermarks

The current TX watermark settings cause the driver to stop queues too
frequently. Among other things, this can have a negative impact on WMM
prioritization, since mac80211 sorts pending packets by their ACs.

Fix this by increasing the high watermark to 256 packets. Increase the
low watermark to 32 to minimize periods with queues being stopped.

Signed-off-by: Ido Yariv <ido@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>