usb: dwc2: gadget: Add dwc2_gadget_read_ep_interrupts function

Reads and returns interrupts for given endpoint, by masking epint_reg
with corresponding mask.

Tested-by: John Keeping <john@metanate.com>
Signed-off-by: Vardan Mikayelyan <mvardan@synopsys.com>
Signed-off-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>