e1000e: normalize usage of serdes_has_link

Cosmetic change to use struct e1000_mac_info.serdes_has_link
consistently as the 'bool' that it's declared as.

No functional change.

Signed-off-by: Alex Chiang <achiang@hp.com>
Acked-by: Jeff Kirsher <Jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>