s390: char: make slp_ctl explicitly non-modular

The Makefile currently controlling compilation of this code is obj-y,
meaning that it currently is not being built as a module by anyone.

Lets remove the couple traces of modular usage, so that when reading
the driver there is no doubt it is builtin-only.

Since module_misc_device translates to device_initcall in the non-modular
case, the init ordering remains unchanged with this commit.

Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Cc: linux-s390@vger.kernel.org
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Acked-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>