Staging: octeon: depends on NETDEVICES

OCTEON_ETHERNET should depend on NETDEVICES.

Fixes this kconfig warning:

warning: (NET_DSA && NET && EXPERIMENTAL && NETDEVICES && !S390 || ...
|| OCTEON_ETHERNET && STAGING && !STAGING_EXCLUDE_BUILD && CPU_CAVIUM_OCTEON) selects PHYLIB which has unmet direct dependencies (!S390 && NETDEVICES)

Reported-by: Arnaud Lacombe <lacombar@gmail.com>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: support@caviumnetworks.com
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>