Staging: dgnc: release the lock before testing for nullity

The refactoring intrduced in
c84a083b ("Staging: dgnc: Use goto for spinlock release before return")
inverts the order in which the lock is released and ld is tested for nullity.

This patch restores the execution flow.

Fixes: c84a083b ("Staging: dgnc: Use goto for spinlock release before return")
Signed-off-by: Quentin Lambert <lambert.quentin@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>