ath10k: set max mtu to 1500 for sdio chip

For sdio chip, the max credit size in firmware is 1556, the 1556
include payload, ieee80211 header, htt header, htc header. So it
need to set the max mtu to 1500 to forbidden TX packet which exceed
1500 form application.

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>