mt76: set fops_tx_stats.owner to THIS_MODULE

If THIS_MODULE is not set, the module would be removed while debugfs is
being used.
It eventually makes kernel panic.

Fixes: e57b7901

 ("mt76: add mac80211 driver for MT7915 PCIe-based chipsets")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>