greybus: define gb_connection_err()

greybus: define gb_connection_err()

Define a function that prints error information about a Greybus
connection in a standard format.  This adopts the convention that
[M:I:C] represents the "path" the connection represents--specifying
the module id, the interface number on that module, and the
connection id on that interface.

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