usb: gadget: r8a66597-udc: use devm_clk_get() to get clock

Change to using the devm_clk_get() to get the clock and
have it automatically freed on exit.

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Felipe Balbi <balbi@ti.com>