staging: bcm2835-audio: off by one in snd_bcm2835_playback_open_generic()

The > should be >= otherwise we write beyond the end of the array when
we do:

	chip->alsa_stream[idx] = alsa_stream;

Fixes: 23b028c8 ("staging: bcm2835-audio: initial staging submission")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>