staging: bcm2835-audio: Code refactoring of vchiq accessor codes

This is a cleanup and code refactoring in bcm2835-vchiq.c.

The major code changes are to provide local helpers for easier use of
lock / unlock, and message passing with/without response wait.  This
allows us to reduce lots of open codes.

Also, the max packet is set at opening the stream, not at each time
when the write gets called.
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Tested-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>