usb: dwc3: Fix handling of ISOC start transfer

A transfer on an isochronous endpoint needs special handling if
it was queued after the interval has expired. This case is
checked by testing bit 13 when a START TRANSFER command fails,
and returning -EAGAIN to indicate to the caller to end the transfer
and try again on the next interval. However, the test itself is
obscured because the function will simply return -EINVAL if any
of bits 15:12 are set. Fix this by correctly reordering the checks.

This was caused by an incorrect merge conflict resolution between
commit 561ba0fe ("usb: dwc3: gadget: return error if command
sent to DEPCMD register fails") and commit ef1891fa ("USB:
dwc3: Fix issues in isochronous endpoint handling").

Change-Id: Id1ec2c030efb94bac435172d7f030e68c7e10535
Signed-off-by: Jack Pham <jackp@codeaurora.org>