mailbox: bcm2708-vcio: Check the correct status register before writing

With the VC reader blocked and the ARM writing, MAIL0_STA reads
empty permanently while MAIL1_STA goes from empty (0x40000000)
to non-empty (0x00000001-0x00000007) to full (0x80000008).

Suggested-by: Phil Elwell <phil@raspberrypi.org>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>