usb: renesas_usbhs: Fix fifo unclear in usbhsf_prepare_pop

This patch fixes an issue for control write. When usbhsf_prepare_pop()
is called after this driver called a gadget setup function, this controller
doesn't receive the control write data. So, this patch adds a code to clear
the fifo for control write in usbhsf_prepare_pop().

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