igb: modify register test for i350 to reflect read only bits in RDLEN/TDLEN

The registers for RDLEN/TDLEN on i350 have the first 7 bits as read only.
This is a change from previous hardware in which it was only the first 4
bits that were read only.

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>