ath10k: reduce transmit msdu count

Reduce the transmit MSDU count for SDIO, to match with the descriptors
as used by the firmware. This also acts as a high watermark level for
transmit. Too many packets to the firmware results in transmit overflow
interrupt.

It only affect SDIO chip, it will not cause functionaly changes to
other hardware.

Tested with QCA6174 SDIO with firmware
WLAN.RMH.4.4.1-00005-QCARMSWP-1.

Signed-off-by: Alagu Sankar <alagusankar@silex-india.com>
Signed-off-by: Wen Gong <wgong@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>