usb: gadget: renesas_usbhs: remove usbhsh_request list

mod_host had usbhsh_request active/free list.
it was almost meaningless, and vainly complicated.
This patch remove it.

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