greybus: connection: clean up control-disconnected helper

Rename helper to the more descriptive
gb_connection_control_disconnected().

Use u16 for cport number, remove redundant cport number from warning
message, and shorten a long line.

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