usb: gadget: renesas_usbhs: each pipe hold maxpacket size

Current renesas_usbhs pipe accessed DCPMAXP/PIPEMAXP register
to get own maxpacket size every time.
But maxpacket size isn't changed after pipe start,
and register access is too slow.

This patch adds new maxp variable to keep own maxpacket.
And un-used function are removed.

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