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: dc35a10f ("net: mvneta: bm: add support for hardware buffer management")
Signed-off-by: David S. Miller <davem@davemloft.net>