usb: dwc3: keystone: check return value

Function devm_clk_get() returns an ERR_PTR when it fails. However, in
function kdwc3_probe(), its return value is not checked, which may
result in a bad memory access bug. This patch fixes the bug.

Signed-off-by: Pan Bian <bianpan2016@163.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>