Fix possible out-of-bound access in ieee80211_get_rate_duration routine
as reported by the following UBSAN report:

UBSAN: array-index-out-of-bounds in net/mac80211/airtime.c:455:47
index 15 is out of range for type 'u16 [12]'
CPU: 2 PID: 217 Comm: kworker/u32:10 Not tainted 6.1.0-060100rc3-generic
Hardware name: Acer Aspire TC-281/Aspire TC-281, BIOS R01-A2 07/18/2017
Workqueue: mt76 mt76u_tx_status_data [mt76_usb]
Call Trace:
 <TASK>
 show_stack+0x4e/0x61
 dump_stack_lvl+0x4a/0x6f
 dump_stack+0x10/0x18
 ubsan_epilogue+0x9/0x43
 __ubsan_handle_out_of_bounds.cold+0x42/0x47
ieee80211_get_rate_duration.constprop.0+0x22f/0x2a0 [mac80211]
 ? ieee80211_tx_status_ext+0x32e/0x640 [mac80211]
 ieee80211_calc_rx_airtime+0xda/0x120 [mac80211]
 ieee80211_calc_tx_airtime+0xb4/0x100 [mac80211]
 mt76x02_send_tx_status+0x266/0x480 [mt76x02_lib]
 mt76x02_tx_status_data+0x52/0x80 [mt76x02_lib]
 mt76u_tx_status_data+0x67/0xd0 [mt76_usb]
 process_one_work+0x225/0x400
 worker_thread+0x50/0x3e0
 ? process_one_work+0x400/0x400
 kthread+0xe9/0x110
 ? kthread_complete_and_exit+0x20/0x20
 ret_from_fork+0x22/0x30

Fixes: db3e1c40 ("mac80211: Import airtime calculation code from mt76")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Acked-by: Toke Høiland-Jørgensen <toke@toke.dk>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>