net: liquidio: fix mixed module-builtin object

With CONFIG_LIQUIDIO=m and CONFIG_LIQUIDIO_VF=y (or vice versa),
$(common-objs) are linked to a module and also to vmlinux even though
the expected CFLAGS are different between builtins and modules.

This is the same situation as fixed by commit 637a642f5c ("zstd:
Fixing mixed module-builtin objects").

Introduce the new module, liquidio-core, to provide the common functions
to liquidio and liquidio-vf.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/cavium/Kconfig

@@ -62,6 +62,9 @@ config CAVIUM_PTP
 	  Precision Time Protocol or other purposes.  Timestamps can be used in
 	  BGX, TNS, GTI, and NIC blocks.
 
+config LIQUIDIO_CORE
+	tristate
+
 config LIQUIDIO
 	tristate "Cavium LiquidIO support"
 	depends on 64BIT && PCI
@@ -69,6 +72,7 @@ config LIQUIDIO
 	depends on PTP_1588_CLOCK_OPTIONAL
 	select FW_LOADER
 	select LIBCRC32C
+	select LIQUIDIO_CORE
 	select NET_DEVLINK
 	help
 	  This driver supports Cavium LiquidIO Intelligent Server Adapters
@@ -92,6 +96,7 @@ config LIQUIDIO_VF
 	tristate "Cavium LiquidIO VF support"
 	depends on 64BIT && PCI_MSI
 	depends on PTP_1588_CLOCK_OPTIONAL
+	select LIQUIDIO_CORE
 	help
 	  This driver supports Cavium LiquidIO Intelligent Server Adapter
 	  based on CN23XX chips.