usb: dwc3: drop HAVE_CLK dependency from Exynos glue layer

commit 93abe8e4 (clk: add non CONFIG_HAVE_CLK routines)
added clk API stubs when !defined(CONFIG_HAVE_CLK).

This allows us to remove the HAVE_CLK dependency from
Exynos' glue layer and let it compile always.

Signed-off-by: Felipe Balbi <balbi@ti.com>