Restock the RX queue when there are a lot of unused frames so that the
RX ring buffer doesn't overrun, causing a ucode assertion.  Backport of
patch "iwlwifi: fix ucode assertion for RX queue overrun".
Signed-off-by: NDan Williams <dcbw@redhat.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>