e1000e: fix check for manageability on ICHx/PCH

Do not check for all the bits in E1000_FWSM_MODE_MASK when checking for
manageability on 82577/82578; only check if iAMT is enabled.  Both of the
manageability checks (for 82577/82578 and ICHx) must check the firmware
valid bit too since the other bits are only valid when the latter is set.

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Tested-by: Jeff Pieper <jeffrey.e.pieper@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>