iwl3945: port allow skb allocation in tasklet patch

Port "iwlcore: Allow skb allocation from tasklet." to 3945

If RX queue becomes empty then we need to restock the queue from
tasklet to prevent ucode from starving. A caller to iwl_rx_allocate
will decide if allocated buffer should come from GFP_ATOMIC or
GFP_KERNEL.

Signed-off-by: Abhijeet Kolekar <abhijeet.kolekar@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>