usb: isp1760: Use the gpio descriptor API

Switching to the managed gpio descriptor API simplifies both error and
cleanup code paths (by removing the need to free the gpio) and runtime
code (by removing manual handling of the active low flag). It also
permits handling the reset gpio entirely from within the HCD code,
sharing it between the different glue layers.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>