KVM: arm64: Return early from read_id_reg() if register is RAZ

If read_id_reg() is called for an ID register which is Read-As-Zero (RAZ),
it initializes the return value to zero, then goes through a list of
registers which require special handling before returning the final value.

By not returning as soon as it checks that the register should be RAZ, the
function creates the opportunity for bugs, if, for example, a patch changes
a register to RAZ (like has happened with PMSWINC_EL0 in commit
11663111), but doesn't remove the special handling from read_id_reg();
or if a register is RAZ in certain situations, but readable in others.

Return early to make it impossible for a RAZ register to be anything other
than zero.

Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20211011105840.155815-2-alexandru.elisei@arm.com