wifi: iwlegacy: Check rate_idx range after addition

Limit rate_idx to IL_LAST_OFDM_RATE for 5GHz band for thinkable case
the index is incorrect.

Reported-by: Fedor Pchelkin <pchelkin@ispras.ru>
Reported-by: Alexei Safin <a.safin@rosa.ru>
Signed-off-by: Stanislaw Gruszka <stf_xl@wp.pl>
Reviewed-by: Fedor Pchelkin <pchelkin@ispras.ru>
Link: https://patch.msgid.link/20250525144524.GA172583@wp.pl


Signed-off-by: Johannes Berg <johannes.berg@intel.com>