mwifiex: change transmit buffer size for 8897

Currently default Tx buffer size configured to firmware is 2K
for all chipsets. This patch changes it to 4K for SD/PCIe/USB
8897 chipsets as per firmware requirements.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>