net: sun8i_emac: Name magic bits and simplify read-modify-write calls

The EMAC driver contains a lot of magic bits, although the manuals
and the Linux driver have all names for them.

Define those names and use them when programming the registers.
Also this replaces a lot of readl/mask/writel operations with the much
easier-to-read setbits_le32() macro.

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>