mwifiex: modify SDIO aggregation Tx/Rx buffer size

The SDIO aggregation buffer size has been modified to an optimum
value which gives good throughput results.

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>