drivers/net/ethernet/e1000e: check return value of e1e_rphy()

e1e_rphy() could return error value when reading PHY register, which
needs to be checked.

Signed-off-by: Li Zhong <floridsleeves@gmail.com>
Acked-by: Sasha Neftin <sasha.neftin@intel.com>
Tested-by: Naama Meir <naamax.meir@linux.intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>