greybus: loopback: ensure count decrement happens before sysfs_remove_groups

This patches fixes a case where gb_dev.count is decremented too late in the
exit() routine.

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