staging: bcm2835-audio: Need to judge the return value of vchi_msg_dequeue in audio_vchi_callback

If vchi_msg_dequeue return -1, variable m is not assigined,
need to return.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: zhengbin <zhengbin13@huawei.com>
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/1570520515-2186-2-git-send-email-zhengbin13@huawei.com


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