only check RCTL_EN in e1000_can_receive()

only check RCTL_EN in e1000_can_receive()

e1000_receive() has code to raise a receive overflow interrupt when the receive
buffer head and tail match. However, with the present implementation of
e1000_can_receive(), this code is unreachable -- and etherboot breaks as a
result.

Signed-off-by: Charles Duffy <charles_duffy@messageone.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@4987 c046a42c-6fe2-441c-8c8c-71466251a162