e1000e: separate out PHY statistics register updates

The 82577/82578 parts have half-duplex statistics in PHY registers.  These
need only be read when in half-duplex and should all be read at once rather
than one at a time to prevent excessive cycles of acquiring/releasing the
PHY semaphore.

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>