greybus: count rather than list protocol users

greybus: count rather than list protocol users

We don't really need a list of protocol users, we can just keep
track of how many there are.  Get rid of the list and use a count
instead.

Also, have gb_protocol_get() return the protocol rather than assigning
a passed-in connection pointer's protocol.  Make a comparable change
to the gb_protocol_put() interface.

Get rid of gb_protocol_find() (the version that locks), because it
is no longer needed.

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