ath10k: drop the TX packet which size exceed credit size for sdio

ath10k: drop the TX packet which size exceed credit size for sdio

sdio chip use DMA buffer to receive TX packet from ath10k, and it has
limitation of each buffer, if the packet size exceed the credit size,
it will trigger error in firmware.

Tested with QCA6174 SDIO with firmware
WLAN.RMH.4.4.1-00017-QCARMSWP-1.

Signed-off-by: Wen Gong <wgong@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200422084719.3479-1-wgong@codeaurora.org