usb: dwc2: gadget: fix debug loop limits

< 15 check doesn't show debug information for endpoint 15.
It is possible to have less than 15 endpoints so use limit
provided by hardware configuration.

Tested-by: Robert Baldyga <r.baldyga@samsung.com>
Acked-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Mian Yousaf Kaukab <yousaf.kaukab@intel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>