KVM: PPC: Book3S: Remove useless checks in 'release' method of KVM device

There is no need to test for the device pointer validity when releasing
a KVM device. The file descriptor should identify it safely.

Fixes: 2bde9b3e

 ("KVM: Introduce a 'release' method for KVM devices")
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>