ixgbe/base: restrict EEE to X550em KR backplane

ixgbe/base: restrict EEE to X550em KR backplane

This patch sets the setup_EEE function pointer to NULL for the
interfaces which do not support EEE (Energy Efficient Ethernet).
Currently only the KR backplane interface (0x15AB) supports EEE.
Setting this pointer to NULL prevents EEE registers from being
incorrectly modified and gives base drivers a flag to check for
EEE support.

Signed-off-by: Wenzhuo Lu <wenzhuo.lu@intel.com>
Acked-by: Helin Zhang <helin.zhang@intel.com>