usb: dwc2: gadget: Fix coverity issue

When _param is unsigned and the minimum value of range is 0, it gives
the following warning:
COVERITY NO_EFFECT: This less-than-zero comparison of an unsigned value
is never true.

Converting ._param to int to avoid this warning.

Signed-off-by: Grigor Tovmasyan <tovmasya@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>