e1000e: cleanup functions that clear hardware statistics

The e1000_clear_hw_cntrs_*() functions read the registers to clear them.
There is no reason to save the register contents so the temp variable can
be removed.

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>