usb: dwc3: qcom: Improve error handling

dwc3_qcom_clk_init() is called with of_count_phandle_with_args() as an
argument.  If of_count_phandle_with_args() returns an error, the number
of clocks will be a negative value and will lead to undefined behaviour.

Ensure we check for an error before attempting to blindly use the value.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>