w5100: remove MMIO support

w5100: remove MMIO support

This driver supports both SPI and MMIO based register access, but only
the former has devicetree support. While MMIO mode would have worked
with old-style board files, those have never defined such a device
upstream.

Remove the MMIO mode, leaving SPI as the only way to use this driver,
but leave it in two loadable modules. More cleanups can be done by
combining the two into one file.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://patch.msgid.link/20260505180459.1247690-1-arnd@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>