drivers/net/ethernet/marvell/mvmdio.c: Fix non OF case

drivers/net/ethernet/marvell/mvmdio.c: Fix non OF case

Orion5.x systems are still using machine files and not device-tree.
Commit 96cb4342382290c9 ("net: mvmdio: allow up to three clocks to be
specified for orion-mdio") has replaced devm_clk_get() with of_clk_get(),
leading to a oops at boot and not working network, as reported in
https://lists.debian.org/debian-arm/2019/07/msg00088.html and possibly in
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=908712.

Link: https://lists.debian.org/debian-arm/2019/07/msg00088.html
Fixes: 96cb4342382290c9 ("net: mvmdio: allow up to three clocks to be specified for orion-mdio")
Signed-off-by: Arnaud Patard <arnaud.patard@rtp-net.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>