net/marvell: fix Wvoid-pointer-to-enum-cast warning

'type' is an enum, thus cast of pointer on 64-bit compile test with
W=1 causes:

  mvmdio.c:272:9: error: cast to smaller integer type 'enum orion_mdio_bus_type' from 'const void *' [-Werror,-Wvoid-pointer-to-enum-cast]

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Tested-by: Simon Horman <horms@kernel.org> # build-tested
Signed-off-by: David S. Miller <davem@davemloft.net>