hw/mips/jazz: Move the NIC init code into a separate function

hw/mips/jazz: Move the NIC init code into a separate function

The mips_jazz_init() function is already quite big, so moving
away some code here can help to make it more understandable.
Additionally, by moving this code into a separate function, the
next patch (that will refactor the for-loop around the NIC init
code) will be much shorter and easier to understand.

Message-ID: <20230913160922.355640-2-thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>