V4L/DVB (13327): em28xx: fix alt modprobe parameter

It seems that some patch broke alt modprobe parameter. Fix it to allow
changing alternate interfaces during module load and at runtime.

If changed during runtime, you'll need to stop a and restart stream for
the parameter to be used.

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