usb: dwc3: gadget: Fix PCM1 for ISOC EP with ep->mult less than 3

For isochronous endpoints with ep->mult less than 3, PCM1 value of
trb->size in set incorrectly.
For ep->mult = 2, this is set to 0/-1 and for ep->mult = 1, this is
set to -2. This is because the initial mult is set to ep->mult - 1
instead of 2.

Signed-off-by: Manu Gautam <mgautam@codeaurora.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>