net: mvneta: bm: fix dependencies again

I tried to fix this before, but my previous fix was incomplete
and we can still get the same link error in randconfig builds
because of the way that Kconfig treats the

	default y if MVNETA=y && MVNETA_BM_ENABLE

line that does not actually trigger when MVNETA_BM_ENABLE=m,
unlike I intended.
Changing the line to use MVNETA_BM_ENABLE!=n however has
the desired effect and hopefully makes all configurations
work as expected.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 019ded3a ("net: mvneta: bm: clarify dependencies")
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>