usb: renesas_usbhs: divide data transfer functions

DMAEngine will be supported to this driver in the future.
Then, both PIO and DMA data transfer method should be supported.
But, the transfer function can returns the result immediately
in PIO version, but it can't in DMA version.
This patch divides data transfer functions into top/bottom half
in preparation for DMAEngine support.

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