mt76: remove rx_page_lock

We can not run mt76u_alloc_buf() concurently, rx_tasklet is stooped
when mt76u_submit_rx_buffers(). We can remove rx_page_lock.

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