iwlwifi: rs-fw: enable STBC in he correctly

In the HE phy capabilities IE there are 2 bits to signal support for
STBC in bandwidths of 80Mhz or less, and of 160Mhz.
Use these bits to determine STBC support if this IE exists.

Signed-off-by: Naftali Goldstein <naftali.goldstein@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>