wifi: iwlegacy: replace BUG_ON() with WARN_ON() on num_stations check

wifi: iwlegacy: replace BUG_ON() with WARN_ON() on num_stations check

BUG_ON() for il->num_stations < 0 can happen in real word, see
https://bugzilla.kernel.org/show_bug.cgi?id=221733

Replace BUG_ON() with WARN_ON() (and reset the counter to 0) to
do not put whole system to inconsistent state on the condition.

Also allocate debugfs buffer for all stations (32 or 25)
to do not use num_stations since it might not be right.

Signed-off-by: Stanislaw Gruszka <stf_xl@wp.pl>
Link: https://patch.msgid.link/20260724095545.33647-1-stf_xl@wp.pl
[clarify commit message wrt. debugfs buffer]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>