V4L/DVB: media/radio: fix copy_to_user to user handling

copy_to/from_user() returns the number of bytes remaining to be copied
but the code here was testing for negative returns.  I modified it to
return -EFAULT.  These functions are called from si4713_s_ext_ctrls() and
that only tests for negative error codes.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>