staging: vchiq_arm: Use define for doorbell irq

The interrupt handler uses a magic number to check that the doorbell
was rung. Better replace this number with official Broadcom define.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Link: https://lore.kernel.org/r/1621105859-30215-9-git-send-email-stefan.wahren@i2se.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>