iwlwifi: size firmware flags memory correctly

Instead of relying on a hard-coded constant of a maximum of 64 API and
capability bits, add a new enum value after the others that will then
always track the number of used bits in the API/capabilities. We thus
no longer need to maintain the maximum number, and on 32-bit platforms
even (currently) reduce the number of bits kept in memory.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>