ixgbe: Correct X550EM_x revision check

The X550EM_x revision check needs to check a value, not just a bit.
Use a mask and check the value. Also remove the redundant check
inside the ixgbe_enter_lplu_t_x550em, because it can only be called
when both the mac type and revision check pass.

Signed-off-by: Mark Rustad <mark.d.rustad@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>