mt76: mt7915: set fops_sta_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: ec9742a8 ("mt76: mt7915: add .sta_add_debugfs support")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>