e1000: Dead variable cleanup

Removal of unused rx_dropped counter.

Removed reference to E1000_CTRL_EXT_CANC which is no longer valid,
replaced with E1000_CTRL_EXT_INT_TIMER_CLR


Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>
Signed-off-by: John Ronciak <john.ronciak@intel.com>