usb: gadget: r8a66597-udc: fix flush fifo handling

The "BCLR" in CFIFOCTR/DnFIFOCTR can flush the fifo of "CPU side" only.
To flush the fifo of "SIE side", we have to use the "ACLRM" in PIPEnCTR.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>