iwlwifi: remove duplicate defines

RX_FREE_BUFFERS and RX_LOW_WATERMARK are currently defined in four places.
Based on how files are included we only need the definition in iwl-fh.h

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Reported-by: Frans Pop <elendil@planet.nl>
Signed-off-by: John W. Linville <linville@tuxdriver.com>