ixgbe: check EEPROM for WOL support for X540 and above

This change aims to simplify the logic we use to determine WOL
support by reading the EEPROM bits for MACs X540 and newer.

Also some cleanups in ixgbe_wol_supported() - changed return type to
bool and removed redundant return variable by simply using return after
the checks.

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>