net: i825xx: 82596: use MMIO accessors for the MVME16x PORT/CA registers

On the MVME16x the two port registers need to be written in order to latch
the correct 32bit value.

The way the code is currently written means that modern GCC can decide to
optimize the writes and break that latching.

Convert the MVME16x specific register accesses to use the proper io
accessors to restore the latching and make the driver functional again.

The Patch was tested by me on real Hardware (Motorola MVME177).
On the Board is a Intel i82596CA 10Mb Ethernet Chip Stepping C.

Reported-by: Denis Ahrens <denis@h3q.com>
Signed-off-by: Daniel Palmer <daniel@0x0f.com>
Tested-by: Denis Ahrens <denis@h3q.com>
Link: https://patch.msgid.link/20261025024514.802862-1-daniel@0x0f.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>