usb: renesas_usbhs: (cosmetic) simplify list operations

list.h already provide helpers to find the first entry and to move list
nodes to the tail of another list. This patch simply uses those helpers,
no functional changes.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>