net: pcs: rzn1-miic: Make switch mode mask SoC-specific

net: pcs: rzn1-miic: Make switch mode mask SoC-specific

Move the hardcoded switch mode mask definition into the SoC-specific
miic_of_data structure. This allows each SoC to define its own mask
value rather than relying on a single fixed constant. For RZ/N1 the
mask remains GENMASK(4, 0).

This is in preparation for adding support for RZ/T2H, where the
switch mode mask is GENMASK(2, 0).

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Tested-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://patch.msgid.link/20250910204132.319975-7-prabhakar.mahadev-lad.rj@bp.renesas.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>