net: mdio: Move mdiobus_scan() within file

No functional change, just place it earlier in preparation for some
refactoring.

While at it, correct the comment format and one typo.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>