staging: vchiq_arm: remove hardcoded buffer length

We better use sizeof instead of hardcoding buffer length multiple
times. This make it easier to increase the buffer in the future.
In order to keep below 80 chars limit make the variable name shorter.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>