iwlwifi: queue functions cleanup

This patch moves
1. iwl_queue_inc_wrap and int iwl_queue_dec_wrap
into  iwl-helpers.h these two functions are identical to common
for both iwl3956 and iwl4965
2. renames x2_queue_used to iwl3945_x2_queue_used
3. exports iwl3945_queue_space out of iwl3964-base.c

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>