net: encx24j600: constify struct regmap_bus/regmap_config

`regmap_encx24j600`, `phycfg` and `phymap_encx24j600` are not modified
and can be declared as const to move their data to a read-only section.

Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Link: https://patch.msgid.link/20240703-net-const-regmap-v1-3-ff4aeceda02c@gmail.com


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