iwlwifi: mvm: prepare for station count change

In a000 devices we will support up to 32 stations.
The max station define is used also for invalid station marking
which makes finding usages of actual maximum station pretty hard
to sort through - change it to be a different define in order
to make future changes easier.
Use also ARRAY_SIZE intead of define when possible.
Do not move yet to 32 stations until firmware do it though.

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