usb: dwc3: exynos: Remove local variable for clock from probe

There's no need to keep one local variable for clock, and
then assign the same to 'clk' member of dwc3_exynos.
Just cleaning it up.

Signed-off-by: Vivek Gautam <gautam.vivek@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>