net: mdio: mscc-miim: replace magic numbers for the bus reset

Replace the magic numbers by macros which are already defined. It seems
the original commit missed to use them.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>