usb: misc: usb3503: Clean up on driver unbind

The driver should clean up after itself by unpreparing the clock when it
is unbound.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>