kvm: selftests: aarch64: fix assert in gicv3_access_reg

The val argument in gicv3_access_reg can have any value when used for a
read, not necessarily 0.  Fix the assert by checking val only for
writes.

Signed-off-by: Ricardo Koller <ricarkol@google.com>
Reported-by: Reiji Watanabe <reijiw@google.com>
Cc: Andrew Jones <drjones@redhat.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220127030858.3269036-2-ricarkol@google.com