greybus: record a gbuf's destination CPort id

Rather than indicating whether a gbuf is intended for outbound data,
record its destination CPort id.  That's what's really needed by
the ES1 host driver.  Use CPORT_ID_BAD when the buffer is intended
for inbound data.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>