e1000e: perform 10/100 adaptive IFS only on parts that support it

e1000e: perform 10/100 adaptive IFS only on parts that support it

Adaptive IFS which involves writing to the Adaptive IFS Throttle register
was being done for all devices supported by the driver even though it is
not supported (i.e. the register doesn't even exist) on some devices.  The
feature is supported on 8257x/82583 and ICH/PCH based devices, but not
on ESB2.

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>