mt76: mt7615: Make mt7615_irq_handler static

Fix sparse warning:

drivers/net/wireless/mediatek/mt76/mt7615/pci.c:37:13:
 warning: symbol 'mt7615_irq_handler' was not declared. Should it be static?

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>