s390/ctlreg: return old register contents when changing bits

Change local_ctl_set_bit() and local_ctl_clear_bit() so they return the
previous value of the to be changed control register. This is useful if a
bit is only changed temporarily and the previous content needs to be
restored.

Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>