greybus: loopback: hold a coarse lock while init/exit run

This patch holds gb_dev.mutex for the duration of init and exit to reduce
complexity while ensuring that init and exit run atomically with respect
to slave threads @ gb_loopback_fn().

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>