CONFIG_NET_SWITCHDEV is a bool option. Change the ifeq conditional to
the standard obj-$(CONFIG_NET_SWITCHDEV) form.

Use obj-y in net/switchdev/Makefile because Kbuild visits this Makefile
only when CONFIG_NET_SWITCHDEV=y.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Link: https://lore.kernel.org/r/20210125231659.106201-3-masahiroy@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>