bcm2836-mbox: last_tx_done should wait for handling on remote side

Previous code tried to verify if VPU->ARM (MAIL0) receiving queue is
empty after submitting message to ARM->VPU (MAIL1) queue. This patch
change mailbox used in last_tx_done to MAIL1.

Based on IRC advice from Ross Oldfield.

Signed-off-by: Piotr Król <piotr.krol@3mdeb.com>