net: Add missing dependencies on NETDEVICES

ETRAX_ETHERNET selects ETHERNET and MII, which depend on NETDEVICES.
I don't think anything should select NETDEVICES, so make it a
dependency.  It also doesn't need to select or depend on ETHERNET,
which has nothing to do with the Ethernet library functions.

BPCTL selects MII, which depends on NETDEVICES.  But everything in the
drivers/staging/silicom directory is related to net devices, so make
NET_VENDOR_SILICOM depend on NETDEVICES and remove the now-redundant
dependencies on NET.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>