usb: dwc3: gadget: fix ISOC TRB type on unaligned transfers

When chaining ISOC TRBs together, only the first ISOC TRB should be of
type ISOC_FIRST, all others should be of type ISOC. This patch fixes
that.

Fixes: c6267a51 ("usb: dwc3: gadget: align transfers to wMaxPacketSize")
Cc: <stable@vger.kernel.org> # v4.11+
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>