usb: gadget: r8a66597-udc: add function for external controller

R8A66597 has the pin of WR0 and WR1. So, if one write-pin of CPU
connects to the pins, we have to change the setting of FIFOSEL
register in the controller. If we don't change the setting,
the controller cannot send the data of odd length.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>