usb: renesas_usbhs: Add a function to write the UGCTRL2 register

To cleanup the code, this patch adds a function to write the UGCTRL2
register.

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