greybus: begin abstracting connection operations

This is part 1 of abstracting the connection operations into a set
of methods.  This will avoid some big switch statements, but more
importantly this will be needed for supporting multiple versions of
each protocol.

For now only two methods are defined.  The init method is used
to set up the device (or whatever the CPort represents) and the exit
method tears it down.  There may need to be additional operations
added in the future, and once versioning is used we might stash
the version number in this structure as well.

The next patch adds dynamic registratration of these protocol
handlers, and will do away with the switch statement now found
in gb_connection_init().

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>
uart-gb: mark some functions static

Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>