net: mvneta: Only disable mvneta_bm for 64-bits

Actually only the mvneta_bm support is not 64-bits compatible.
The mvneta code itself can run on 64-bits architecture.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Tested-by: Marcin Wojtas <mw@semihalf.com>
Signed-off-by: David S. Miller <davem@davemloft.net>