3c59x: Fix call to mdio_sync() with the wrong argument

commit a095cfc4
"3c59x: Specify window explicitly for access to windowed registers"
changed the first parameter to mdio_sync(), from a pointer to the
register mapping, to a pointer to the vortex_private structure,
and changed all but one of the call sites.  Fix that last one.

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