staging: greybus: loopback: Hold per-connection mutex across operations

Commit d9fb3754 ("greybus: loopback: Relax locking during loopback
operations") changes the holding of the per-connection mutex to be less
restrictive because at the time of that commit per-connection mutexes were
encapsulated by a per-driver level gb_dev.mutex.

Commit 8e1d6c33 ("greybus: loopback: drop bus aggregate calculation")
on the other hand subtracts the driver level gb_dev.mutex but neglects to
move the mutex back to the place it was prior to commit d9fb3754
("greybus: loopback: Relax locking during loopback operations"), as a
result several members of the per connection struct gb_loopback are racy.

The solution is restoring the old location of mutex_unlock(&gb->mutex) as
it was in commit d9fb3754 ("greybus: loopback: Relax locking during
loopback operations").

Fixes: 8e1d6c33 ("greybus: loopback: drop bus aggregate calculation")

Signed-off-by: Bryan O'Donoghue <pure.logic@nexus-software.ie>
Reviewed-by: Johan Hovold <johan@kernel.org>
Cc: Alex Elder <elder@kernel.org>
Cc: Mitch Tasman <tasman@leaflabs.com>
Cc: greybus-dev@lists.linaro.org
Cc: devel@driverdev.osuosl.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>