staging/octeon: Fix build error without CONFIG_NETDEVICES

While do COMPILE_TEST build without CONFIG_NETDEVICES,
we get Kconfig warning:

WARNING: unmet direct dependencies detected for PHYLIB
  Depends on [n]: NETDEVICES [=n]
  Selected by [y]:
  - OCTEON_ETHERNET [=y] && STAGING [=y] && (CAVIUM_OCTEON_SOC && NETDEVICES [=n] || COMPILE_TEST [=y])

Reported-by: Hulk Robot <hulkci@huawei.com>
Reported-by: Mark Brown <broonie@kernel.org>
Fixes: 171a9bae

 ("staging/octeon: Allow test build on !MIPS")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>