iwlwifi: disable the buggy chain extension feature in HW

commit d012d04e4d6312ea157b6cf19e9689af934f5aa7 upstream.

This feature has been reported to be buggy and enabled by
default. We therefore need to disable it manually.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
[bwh: Backported to 3.2 as instructed: pass bus(trans) to iwl_write_prph()]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>