NE2000: Fix regresssion introduced by e710185a on non AX88796

Move non-inlied functions into specific drivers file
Set get_prom as weak

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Vlad Lungu <vlad@comsys.ro>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>