usb: renesas_usbhs: modify data transfer method

On current driver, main data transfer function was implemented in fifo.c,
but the overall controlling was implementing in mod_gadget.c.
This style is not useful to support host and DMAEngine in the future.

But the interrupt for data transfer cannot separate easily for now,
because it is deeply related to mod_gadget.

This patch move the overall data transfer method
into fifo.c except interrupt.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>