mt76: move irq handler in mt76x02-lib moudle

Move mt76x02_irq_handler handler in mt76x02_mmio.c in order to be
reused in mt76x0 driver. Move mt76x02_rx_poll_complete routine in
mt76x02-lib module. Moreover remove pci_core.c and mt76x2/trace.{c,h}
since are empty files

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>