greybus: loopback: exit kfree after mutex release

init doesn't have a lock for kzalloc so exit shouldn't have lock with the
corresponding kfree.

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>