igb: loopback bits not correctly cleared from RCTL register

This change forces the bits to 0 by using an &= operation with an inverted
mask of all options instead of using an |= with a value of 0.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>