usb: phy: samsung: Use clk_get to get reference clock

There is no need to use devm_clk_get to get a clock that is being put
at the end of the function.

This patch changes the code getting reference clock to use clk_get
instead of useless in this case devm_clk_get.

Signed-off-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>