usb: dwc3: don't set last bit for ISOC endpoints

According to Synopsys Databook 2.60a, section 8.3.4,
it's stated that:

	The LST bit should be set to 0 (isochronous
	transfers normally continue until the
	endpoint is removed entirely, at which time
	an End Transfer command is used to stop the
	transfer).

This patch makes sure that detail is observed and
fixes a regression with Android Audio playback
caused by recent changes to DWC3 gadget.

Signed-off-by: Janusz Dziedzic <januszx.dziedzic@linux.intel.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>