usb: gadget: at91_udc: Drop uclk clock

Now that at91 system clocks forward set_rate request to their parent we
can remove the uclk clock and directly call clk_set_rate on fclk.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>