ath10k: sdio: reset chip on power_down()

The target device needs to be reset during power_down(), otherwise only the
first power_up() will work. And as ath10k calls power_up() during driver
initialisation the driver would be otherwise unusable.

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>