iwlwifi: mvm: get number of stations from TLV

FW is changing the max number of supported stations. To adapt to the
change we get the max number from the TLV and act according to the new
number.

Signed-off-by: Nathan Errera <nathan.errera@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/iwlwifi.20201008180656.863ab470babc.I393223392f36436663c4e66add03fefe77b74e60@changeid