usb: dwc3: Fix default lpm_nyet_threshold value

The max possible value for DCTL.LPM_NYET_THRES is 15 and not 255. Change
the default value to 15.

Cc: stable@vger.kernel.org
Fixes: 80caf7d2 ("usb: dwc3: add lpm erratum support")
Signed-off-by: Thinh Nguyen <thinhn@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>