mwifiex: do not reconfigure tx buffer size in firmware while associating

It's observed that reconfiguration of tx buffer size before
association can cause data path failure in firmware after
associated. Although this is only found with PCIe cards, but
potentially it could happen with any other interfaces as well.

The tx buffer reconfiguration is not really useful in firmware.
Let's remove it for all interfaces.

Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>