ath9k_htc: Handle storage devices

Some AR7010 based devices are recognized as storage media.
Sending a CD-EJECT command to the device will 'convert' it into
a WLAN device. Do this within the driver itself, removing the
dependancy on an external program (usb_modeswitch).

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