iwlwifi: don't disable SCD chain extension on newer devices

7000 device series have a fix for this hardware feature.
Stop disabling it, and get an improvement in Tx throughput.
This feature allows the scheduler to fetch more frames on
the fly while an A-MPDU is being built - which means that
we can get larger A-MPDU. This, of course, give an
improvement in the Tx throughput.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>