net: ethernet: sun4i-emac: replace magic number with macro

This patch remove magic numbers in sun4i-emac.c and replace with macros
defined in sun4i-emac.h

Signed-off-by: Conley Lee <conleylee@foxmail.com>
Tested-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Link: https://lore.kernel.org/r/tencent_71466C2135CD1780B19D7844BE3F167C940A@qq.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>