xen/arm: Fix ARM build following c/s 11c397c

xen/arm: Fix ARM build following c/s 11c397c

c/s 11c397c broke the ARM build by introducing a common ACCESS_ONCE() which is
different to the definition in smmu.c

The SMMU code included a scalar typecheck, which is worth keeping in the
common case, given ACCESS_ONCE()'s restrictions.  However, express the
typecheck differently so as to avoid Coverity complaints about unused
variables.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Julien Grall <julien.grall@arm.com>