s390/cio: de-duplicate subchannel validation

cio_validate_io_subchannel() and cio_validate_msg_subchannel() are
identical, as the called functions already take care about the
differences between subchannel types.

Just inline the code into the only user,
cio_validate_subchannel(), instead.

Signed-off-by: Pierre Morel <pmorel@linux.vnet.ibm.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Acked-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>