staging: vchiq_arm: Avoid premature message stalls

The constants MAX_COMPLETIONS and MSG_QUEUE_SIZE control
the number of messages that can be outstanding to each client
before the system as a whole stalls. If the numbers are too
small then unnecessary thread switching will occur while
waiting for a (potentially low priority) client thread to
consume some data; badly written clients can even lead to
deadlock.

For services that carry many short messages, 16 messages can
represent a very small amount of data. Since the resources
are small - 16 bytes for a completion, 4 bytes for a message
pointer - increase the limits so they are unlikely to be hit
except in exceptional circumstances.

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