iwlwifi: mvm: check sta_id/drain values in debugfs

The station ID must be valid, if it's out of range then
the array access may crash. Validate the station ID to
the array length, and also validate the drain value even
if that doesn't matter all that much.

Cc: stable@vger.kernel.org
Fixes: 8ca151b5

 ("iwlwifi: add the MVM driver")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>