Revert "serdev: simplify Makefile"

Revert "serdev: simplify Makefile"

This reverts commit 54edb425346a4d5e17f7e54e8c97c0d0eac26315.

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.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>