iwlwifi: mvm: change the check for ADD_STA status

The firmware will return the baid for BA session in the
ADD_STA command response.
This requires masking the check of the status, which is
actually only 8 bits, and not the whole 32 bits.

Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>