ixgbe: fix read-modify-write in x550 phy setup

Replaced an assignment operation with an OR operation.

The variable assignment was overwriting the value read from the PHY
register. The OR operation sets only the intended register bits.

The bits that were being overwritten are reserved, so the assignment had no
functional impact.

Reported by: Shannon Nelson <shannon.nelson@oracle.com>
Signed-off-by: Paul Greenwalt <paul.greenwalt@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>