greybus: add num_cports field to greybus hd

This commit is doing the preparation work in order to get the number of cports
supported from the UniPro IP instead of using a constant defined in a Kconfig
file.

Greybus host device is now holding the cport count, and all the code will now
use this value instead of the constant CPORT_ID_MAX when referring to an AP's
CPort ID.

Signed-off-by: Fabien Parent <fparent@baylibre.com>
[johan: es1 supports 256 cports, minor style changes ]
Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>