greybus: connection: no need to verify connection->protocol

connection->protocol will always be valid in gb_connection_init() as it
is called only from a single routine, after initializing the 'protocol'
field.

No need to check it again.

Reviewed-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Reviewed-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>