net: dsa: tag_ocelot_8021q: fix driver dependency

When the ocelot driver code is in a library, the dsa tag
code cannot be built-in:

ld.lld: error: undefined symbol: ocelot_can_inject
>>> referenced by tag_ocelot_8021q.c
>>>               dsa/tag_ocelot_8021q.o:(ocelot_xmit) in archive net/built-in.a

ld.lld: error: undefined symbol: ocelot_port_inject_frame
>>> referenced by tag_ocelot_8021q.c
>>>               dsa/tag_ocelot_8021q.o:(ocelot_xmit) in archive net/built-in.a

Building the tag support only really makes sense for compile-testing
when the driver is available, so add a Kconfig dependency that prevents
the broken configuration while allowing COMPILE_TEST alternative when
MSCC_OCELOT_SWITCH_LIB is disabled entirely.  This case is handled
through the #ifdef check in include/soc/mscc/ocelot.h.

Fixes: 0a6f17c6 ("net: dsa: tag_ocelot_8021q: add support for PTP timestamping")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20210225143910.3964364-2-arnd@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/dsa/Kconfig

@@ -118,6 +118,8 @@ config NET_DSA_TAG_OCELOT
 
 config NET_DSA_TAG_OCELOT_8021Q
 	tristate "Tag driver for Ocelot family of switches, using VLAN"
+	depends on MSCC_OCELOT_SWITCH_LIB || \
+	          (MSCC_OCELOT_SWITCH_LIB=n && COMPILE_TEST)
 	select NET_DSA_TAG_8021Q
 	help
 	  Say Y or M if you want to enable support for tagging frames with a