iwlwifi: mvm: use correct FIFO length

[ Upstream commit 7126b6f2bbdf8e25f85e7ca6d91d49ea4ce9f6a6 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>