iwlwifi: revert workaround to restore 1000 rx throughput

A workaround in commit c0486b7ccc5 resulted in a 40% drop in receive
throughput in order to fix a transmit problem.  The transmit problem
no longer occurs, so restore the receive throughput.

Signed-off-by: Don Fry <donald.h.fry@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>