staging: bcm2835-audio: replace null with error pointer value

This patch replaces NULL values returned by vc_vchi_audio_init(...) with
error pointer values:
	- Return ERR_PTR(-EINVAL) when too many instances of audio
	  service are initialised
	- Return ERR_PTR(-ENOMEM) when kzalloc fails
	- RETURN ERR_PTR(-EPERM) when vchi connections fail to open

Similarly, a NULL check where vc_vchi_audio_init(...) is called is
replaced by IS_ERR(..)

Signed-off-by: Aishwarya Pant <aishpant@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>