mt76: mt7615: rework IRQ handling to prepare for MSI support

With MSI interrupts, IRQs must not be enabled from within the IRQ handler,
because that can lead to lost events.
Defer IRQ processing to a tasklet, which is also responsible for enabling
IRQs (to avoid race conditions against the handler)

Co-developed-by: Soul Huang <Soul.Huang@mediatek.com>
Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Soul Huang <soul.huang@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>