V4L/DVB (12748): em28xx: implement g_std v4l call

We need to implement the g_std call, or else the default norm always gets
returned, which breaks VBI capturing if you had changed the standard to NTSC
using s_std.

I had temporarily changed the default norm to NTSC so that zvbi-ntsc-cc
wouldn't choke, so now that we are returning the correct value, switch it back
to PAL as the default.

This work was sponsored by EyeMagnet Limited.

Signed-off-by: Devin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>