greybus: connection: move legacy-protocol handling to legacy driver

Move legacy protocol and connection handling to the legacy driver.

Rename the former global functions using a common legacy_ prefix.

Note that all legacy protocols suffer from a connection initialisation
race in that the protocol-specific initialisation, which includes
allocation of protocol-specific state containers, can not happen until
*after* the connection has been enabled. This is a major flaw in the
original design that we can now finally address by converting the legacy
protocol drivers into proper bundle (class) drivers.

Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>