gbsim: Fix greybus protocol -> protocol_id renaming issue

Trying to build gbsim with the latest greybus, I'm hitting the
following build issue:

manifest.c: In function ‘identify_descriptor’:
manifest.c:77:32: error: ‘struct greybus_descriptor_cport’ has no
member named ‘protocol’

Related to the fact that the greybus cport protocol field is now
named protocol_id. This patch fixes it by renaming the usage.

Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Matt Porter <mporter@linaro.org>