staging: bcm2835-audio: Simplify callback structure for write data

The device sends data to the audio devices by sending a message with
the data through VC04_SERVICES/VCHIQ.  This message contains a
callback pointer that is always filled in with the same function.
This is prone to corruption issues.

Instead fill the callback fields with a fixed cookie value to perforam
some validation on the message response and call the handler function
directly instead of through the callback pointer.

Signed-off-by: Michael Zoran <mzoran@crowfest.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>