V4L/DVB (10157): Add USB ID for the Sil4701 radio from DealExtreme

Signed-off-by: Mark Lord <mlord@pobox.com>
Cc: Greg KH <greg@kroah.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
[tobias.lorenz@gmx.net: Code beautifications and documentation added]
Signed-off-by: Tobias Lorenz <tobias.lorenz@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>