e1000: Fix MANC detection for PCIE adapters

Several manageability capability detection parts hinted towards
our code being incomplete for PCI-E. According to spec, we do not
want to poke any MANC bits at all.

Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>