ath10k: change swap mail box check after htc ready

The swap box flag of firmware is not set before htc ready, then it
will not set swap box flag in ath10k driver, and it will let swap
box setting not same between firmware and ath10k driver, then it
will trigger firmware assert failure.

Check the flag and set swap box after htc ready will fix the firmware
assert failure.

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

Signed-off-by: Wen Gong <wgong@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>