net: sun8i_emac: Simplify mdio_read/mdio_write functions

net: sun8i_emac: Simplify mdio_read/mdio_write functions

When preparing the register value for the MDIO command register, we
start with a zeroed register, so there is no need to mask off certain
bits before setting them.
Simplify the sequence, and rename the variable to a more matching
mii_cmd on the way.

Also the open-coded time-out routine can be replaced with a much safer
and easier-to-read call to wait_for_bit_le32().

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Tested-by: Amit Singh Tomar <amittomer25@gmail.com> # Pine64+
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>