KVM: arm64: Make __vcpu_sys_reg() a pure rvalue operand

Now that we don't have any use of __vcpu_sys_reg() as a lvalue,
remove the in-place update, and directly return the sanitised
value.

Reviewed-by: Miguel Luis <miguel.luis@oracle.com>
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20250603070824.1192795-5-maz@kernel.org


Signed-off-by: Marc Zyngier <maz@kernel.org>