greybus: protocol: switch gb_protocol_register() to return an int

We will want to return this value as a return value for module_init()
and bool does not play well with module_init().  So make it a "real"
error value and return int and fix up all callers of the function.

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