usb: dwc2: gadget: check interrupts for all endpoints

Current code does not check endpoint 15 interrupt. Use number
of endpoint configured in hardware instead of the hardcoded value.

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>