V4L/DVB (12376): em28xx: fix V4L2 API compliance: don't expose audio inputs for devices without it

V4L2 API (chapter 1.5) states that:

	Drivers must implement all input ioctls when the device has one
	or more inputs, all output ioctls when the device has one or more outputs.
	When the device has any audio inputs or outputs the driver must set the
	V4L2_CAP_AUDIO flag in the struct v4l2_capability returned by the
	VIDIOC_QUERYCAP ioctl.

So, devices without audio input should return -EINVAL.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>