greybus: loopback: ensure debugfs entires are cleaned up on exit

bdd4bba4 ('greybus/loopback: add module level sys/debug fs data points')
added a debugfs entry attached to gb_dev but omitted the cleanup on gb_init
error and gb_exit. This patchs fixes the missing debugfs_remove().

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