cmd: mdio: Switch to generic helpers when accessing the registers

Switch to use the generic helpers to access the MMD registers so that we
can used the same command also for C45 PHYs, C22 PHYs with direct and
indirect access and PHYs implementing a custom way to access the
registers.

Signed-off-by: Carlo Caione <ccaione@baylibre.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>