usb: dwc3: gadget: Revise setting IOC when no TRB left

To keep the setting of interrupt-on-completion (IOC) when out of TRBs
consistent and easier to read, the caller of dwc3_prepare_one_trb()
will determine if the TRB must have IOC bit set. This also reduces the
number of times we need to call dwc3_calc_trbs_left(). Note that we only
care about setting IOC from insufficient number of TRBs for SG and not
linear requests (because we don't need to split linear requests).

Signed-off-by: Thinh Nguyen <Thinh.Nguyen@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>