mt76x0: remove mt76x0_burst_write_regs()

We don't need to use custom burst write regs via MCU, we can use
generic mt76_wr_copy() for the same purpose.

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