usb: dwc3: gadget: don't force 'LST' always

the LST bit is to be set on the last of a series
of consecutive TRBs. We had a workaround for a
problem where data would get corrupted but that
doesn't happen anymore. It's likely that it was
caused by some FPGA instability during development
phase.

Signed-off-by: Felipe Balbi <balbi@ti.com>