eepro100: Don't allow writing SCBStatus

SCBStatus is readonly, but most drivers which were derived
from the old Linux eepro100.c do a word write to this address
when they want to acknowledge interrupts.

So we have to mask these writes here.

The patch also removes old unused code for status read / write.

Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>