usb: dwc3: Remove impossible check for of_clk_get_parent_count() < 0

The check for < 0 is impossible now that
of_clk_get_parent_count() returns an unsigned int. Simplify the
code and update the types.

Acked-by: Felipe Balbi <balbi@kernel.org>
Cc: <linux-usb@vger.kernel.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>