greybus: fix host-device buffer constraints

Host devices impose buffer-size constraints on Greybus core which are
taken into account when allocating messages.

Make sure to verify these constraints when the host device is allocated,
rather than when the first message is allocated.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>