iwlwifi: fix ucode assertion for RX queue overrun

This patch allows the driver to restock the RX queue early if the RX
queue is almost empty. This will help on avoiding any ucode assert
for the RX overrun problem.

Signed-off-by: Mohamed Abbas <mabbas@linux.intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>