serdev: simplify Makefile

drivers/tty/Makefile has:
	obj-$(CONFIG_SERIAL_DEV_BUS)    += serdev/

so in drivers/tty/serdev/Makefile CONFIG_SERIAL_DEV_BUS is always =y.
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20191212101649.18126-1-u.kleine-koenig@pengutronix.deSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serdev/Makefile

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