usb: dwc3: Use devres to get clocks

Use devres to get clocks and drop explicit clock freeing. No
functional change intended.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Cc: Felipe Balbi <balbi@kernel.org>
Cc: Chris Healy <cphealy@gmail.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-usb@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>