net: fec: Refactor: #define magic constants

Add defines for bits of ECR, RCR control registers, TX watermark etc.

Signed-off-by: Csókás Bence <csokas.bence@prolan.hu>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20240212153717.10023-1-csokas.bence@prolan.hu


Signed-off-by: Jakub Kicinski <kuba@kernel.org>