mwifiex: modify TX buff size for SD8887

FW crash has been observed while running iperf TX with SD8887
devices. This is because of invalid TX buffer setting. SD8887
supports 2K buffer sizes. This patch fixes this issue.

Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>