kvm/ppc/mpic: fix missing unlock in set_base_addr()

Add the missing unlock before return from function set_base_addr()
when disables the mapping.

Introduced by commit 5df554ad
(kvm/ppc/mpic: in-kernel MPIC emulation)

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Alexander Graf <agraf@suse.de>