iwlwifi: don't stop queue in the middle of fragmented packet

This patch avoids stopping queue in the middle of the fragmented packet.
It is required that there will be ~10 (max packet/min fragment) or 16
(4 bits of frag number) free tfds all the time.

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