staging: rtl8712: Improve suspend/resume functionality.

Cancel pending URBs during suspend operation to avoid receiving ESHUTDOWN
in read/write completion callbacks while the device is suspended.

Receiving ESHUTDOWN in read/write completion callbacks will cause the
driver to enter a non-functioning "stopped" state from which the driver is
unable to recover without reloading the module.

Signed-off-by: Hemmo Nieminen <hemmo.nieminen@iki.fi>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>