usb: dwc3: fix bogus test in dwc3_gadget_start_isoc

Zero is a valid value for a microframe number. So remove the bogus
test for non-zero in dwc3_gadget_start_isoc().

Cc: stable@vger.kernel.org
Signed-off-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>