iwlwifi: create disable SCD Tx FIFOs handler

This patch moves disabeling Tx FIFOs in NIC SCD to seperate handlers
in 4965 and 5000 cards.

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