ARM: KVM: fix KVM_CAP_ARM_SET_DEVICE_ADDR reporting

Commit 3401d54696f9 (KVM: ARM: Introduce KVM_ARM_SET_DEVICE_ADDR
ioctl) added support for the KVM_CAP_ARM_SET_DEVICE_ADDR capability,
but failed to add a break in the relevant case statement, returning
the number of CPUs instead.

Luckilly enough, the CONFIG_NR_CPUS=0 patch hasn't been merged yet
(https://lkml.org/lkml/diff/2012/3/31/131/1), so the bug wasn't
noticed.

Just give it a break!

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <cdall@cs.columbia.edu>