usb: gadget: renesas_usbhs: cleanup complicated ureq alloc/free

DCP data/status stage needs ureq to usbhs_pkt_push(),
but sometimes, there is no data stage.
In that case, allocated ureq was not freed,
Current ureq alloc/free pair were difficult to understand.
This patch removed unnecessary/un-understandable ureq alloc
from usbhsh_urb_enqueue(), and create simpler alloc/free pair.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>