ath10k: add peer flush in ath10k_flush for STATION

In the noisy environment, if there are packets in the queue and can't
send out, the suspend timing will be more than 5 seconds due to the wait,
flush the queue to optimize the suspend timing, and let the upper layer to
retry the packets after resume.

Tested with QCA6174 PCI with firmware
WLAN.RM.4.4.1-00109-QCARMSWPZ-1, but this will also affect QCA9377 PCI.
It's not a regression with new firmware releases.

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