greybus: connection: check for duplicate cport ids

Check at connection creation time for an attempt to create a
connection with an interface CPort ID that's the same as one that's
already been created.

Define a new helper function to look for such a duplicate.  The
check for a duplicate is only performed at initialization time,
and CPorts are initialized serially for each bundle, so there's
no need to acquire the list lock for this search.

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