usb: dwc3: Separate field holding multiple properties

dwc->hird_threshold field should store "snps,hird_threshold" property
only and not a combination of multiple properties. Remove the value of
"snps,is-utmi-l1-suspend" property from the field dwc->hird_threshold.

Signed-off-by: Thinh Nguyen <thinhn@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>