V4L/DVB (12750): em28xx: fix unused variable warning

Remove unused variable from when I introduced the g_std() function.

This work was sponsored by EyeMagnet Limited.

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