rtl8139: correctly check the opmode

According to the spec, only when opmode is "Config. Register Write
Enable" could driver write to CONFIG0,1,3,4 and bits 13,12,8 of BMCR.

Currently, we allow modifying to those registers also when 8139 is in
"Auto-load" mode and "93C46 (93C56) Programming" mode. This patch
fixes this.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
