net: mdio: mux-bcm-iproc: Use alignment helpers and SZ_4K

Instead of open coding, use IS_ALIGNED() and ALIGN_DOWN() when dealing
with alignment. Replace also literals with SZ_4K.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Acked-by: Ray Jui <ray.jui@broadcom.com>
Link: https://lore.kernel.org/r/20231229145232.6163-1-ilpo.jarvinen@linux.intel.com


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