staging: bcm2835-audio: Make single vchi handle

The bcm2835_audio_instance object contains the array of
VCHI_SERVICE_HANDLE_T, while the code assumes and uses only the first
element explicitly.  Let's reduce to a single vchi handle for
simplifying the code.
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>