mt76: Off by one in mt76_calc_rx_airtime()

The sband->bitrates[] array has "sband->n_bitrates" elements so this
check needs to be >= instead of > or we could read beyond the end of the
array.

These values come from when we call mt76_register_device():

	ret = mt76_register_device(&dev->mt76, true, mt7603_rates,
				   ARRAY_SIZE(mt7603_rates));

Here sband->bitrates[] is mt7603_rates[] and ->n_bitrates is the
ARRAY_SIZE()

Fixes: 5ce09c1a ("mt76: track rx airtime for airtime fairness and survey")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>