ath9k_htc: Use USB reboot

So, apparently there is a USB reboot command
that the target accepts. Using this instead of
usb_reset_device() fixes the issue of "descriptor read error"
that pops up on repeated load/unload.

Signed-off-by: Sujith <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>