iwlwifi: publish STBC support in HT

Not all chips support STBC so allow this to be
another config parameter per chip type. If STBC
is supported then publish it in the HT caps.

Since 7260/7265/3160 chips support it - set the stbc
support bit.

Signed-off-by: Eyal Shapira <eyal@wizery.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>