usb: dwc3: exynos remove suspend clock unspecified debug message

dwc3-exynos prints debug message when suspend clock is not specified.
The suspend clock is optional and driver can work without it.

This debug message doesn't add any value and leads to confusion and
concern. Remove it.

Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>