staging: vchiq_core: introduce parse_message

The function parse_rx_slots is very longer. So move at least the message
parsing into a separate function to improve readability. In good case
the function returns the message payload length which is necessary to
move to the next message.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Link: https://lore.kernel.org/r/1621105859-30215-19-git-send-email-stefan.wahren@i2se.com


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