usb: dwc3: gadget: rely on sg_is_last() and list_is_last()

sg_is_last() and list_is_last() will encode the
required information for the driver to make
decisions WRT CHN and LST bits.

While at that, also replace '1' with 'true' for
consistency.

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