Revert "serdev: simplify Makefile"

This reverts commit 54edb425.

The offending commit caused serdev core to always be built-in, something
which breaks the build of dependent modules when serdev is being built
as a module:

ERROR: "__serdev_device_driver_register" [drivers/gnss/gnss-ubx.ko] undefined!
...
make[2]: *** [/home/johan/work/omicron/src/linux/scripts/Makefile.modpost:94: __modpost] Error 1
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Cc: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20191218131154.13702-1-johan@kernel.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serdev/Makefile

 # SPDX-License-Identifier: GPL-2.0
 serdev-objs := core.o
 
-obj-y += serdev.o
+obj-$(CONFIG_SERIAL_DEV_BUS) += serdev.o
 
 obj-$(CONFIG_SERIAL_DEV_CTRL_TTYPORT) += serdev-ttyport.o