rtl8139: validate rx ring before receiving packets

Commit ff71f2e8cacefae99179993204172bc65e4303df prevent the possible
crash during initialization of linux driver by checking the operating
mode.This seems too strict as:

- the real card could still work in mode other than normal
- some buggy driver who does not set correct opmode after eeprom
 access

So, considering rx ring address were reset to zero (which could be
safely trated as an address not intened to DMA to), in order to
both letting old guest work and preventing the unexpected DMA to
guest, we can forbid packet receiving when rx ring address is zero.

Tested-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
