usb: m66592-udc buffer management update

This patch updates the m66592-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.

With this patch applied it is possible to use m66592-udc for
both Ethernet and Serial using CONFIG_USB_CDC_COMPOSITE.

Signed-off-by: Magnus Damm <damm@igel.co.jp>
Acked-by: Yoshihiro Shimoda <shimoda.yoshihiro@renesas.com>
Acked-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>