net: mvneta: bm: clarify dependencies

MVNETA_BM has a dependency on MVNETA, so we can only select the former
if the latter is enabled. However, the code dependency is the reverse:
The mvneta module can call into the mvneta_bm module, so mvneta cannot
be a built-in if mvneta_bm is a module, or we get a link error:

drivers/net/built-in.o: In function `mvneta_remove':
drivers/net/ethernet/marvell/mvneta.c:4211: undefined reference to `mvneta_bm_pool_destroy'
drivers/net/built-in.o: In function `mvneta_bm_update_mtu':
drivers/net/ethernet/marvell/mvneta.c:1034: undefined reference to `mvneta_bm_bufs_free'

This avoids the problem by further clarifying the dependency so that
MVNETA_BM is a silent Kconfig option that gets turned on by the
new MVNETA_BM_ENABLE option. This way both the core HWBM module and
the MVNETA_BM code are always built-in when needed.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: dc35a10f68 ("net: mvneta: bm: add support for hardware buffer management")
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/marvell/Kconfig

@@ -40,10 +40,9 @@ config MVMDIO
 
 	  This driver is used by the MV643XX_ETH and MVNETA drivers.
 
-config MVNETA_BM
+config MVNETA_BM_ENABLE
 	tristate "Marvell Armada 38x/XP network interface BM support"
 	depends on MVNETA
-	select HWBM
 	---help---
 	  This driver supports auxiliary block of the network
 	  interface units in the Marvell ARMADA XP and ARMADA 38x SoC
@@ -67,6 +66,15 @@ config MVNETA
 	  driver, which should be used for the older Marvell SoCs
 	  (Dove, Orion, Discovery, Kirkwood).
 
+config MVNETA_BM
+	tristate
+	default y if MVNETA=y && MVNETA_BM_ENABLE
+	default MVNETA_BM_ENABLE
+	select HWBM
+	help
+	  MVNETA_BM must not be 'm' if MVNETA=y, so this symbol ensures
+	  that all dependencies are met.
+
 config MVPP2
 	tristate "Marvell Armada 375 network interface support"
 	depends on MACH_ARMADA_375