e1000e: fix checks for manageability enabled and management pass-through

The mac->arc_subsystem was being incorrectly used to flag whether or not
manageability was enabled when it should only be used to state whether the
ARC (Host interface) subsystem is available on a particular MAC _and_ only
valid when any manageability is enabled. The ARC subsystem is currently
only available on 80003es2lan and 82573 parts supported by the driver.

A new flag, has_fwsm, is introduced to be used when checking if
manageability is enabled but only on parts that acutally have an FWSM
register. While the above parts have an FWSM register, there are other
parts that have FWSM but do not have support for the ARC subsystem,
namely 82571/2 and ICHx/PCH.

And then there are parts that have manageability, but do not have either
FWSM register or support for the ARC subsystem - these are 82574 and 82583.

For 80003es2lan, 82571/2/3 and ICH/PCH parts, this patch makes no
functional changes, it only corrects the usage of the manageability flags.
For 82574 and 82583, it fixes the incorrect accesses of the non-existent
FWSM register and ARC subsystem as well as corrects the check for
management pass-through.

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