ARM: bcm283x mbox: Fix send status register

Before we can send a message to the mailbox we have to check that there
is space to do so. Therefore we poll the status register. But up to now
the wrong status register, the one of mailbox 0, was checked. Fix this
by polling the status regiser of mailbox 1.

Signed-off-by: Fabian Vogt <fvogt@suse.com>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>
[mb: rename registers and update commit message]
Signed-off-by: Matthias Brugger <mbrugger@suse.com>