iwlwifi: mvm: use correct FIFO length

Current FIFO size calculation is wrong for two reasons:
- We access lmac 0 by default
- We don't take 11ax into consideration.
Fix both.

Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>