greybus: use gbuf's destination cport id

If the buffer allocated in the ES1 alloc_gbuf_data() routine is for
outbound data, we are getting the destination CPort id from the
connection.  Switch to using the copy of the destination cport id
we now have in the gbuf instead.

Check for a valid CPort id there only if we're inserting it into
the buffer.

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