usb: r8a66597-udc buffer management update

This patch updates the r8a66597-udc buffer management code.

Use fixed buffers for bulk and isochronous pipes, also make
sure to handle the isochronous-as-bulk case. With fixed buffers
there is no need to keep track of used buffers with bi_bufnum.

Also, this fixes a potential buffer offset problem where the
base offset incorrectly varies with the number of pipes used.

The m66592 driver recently got fixed in a similar way.

Signed-off-by: Magnus Damm <damm@igel.co.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>