V4L/DVB (9350): radio-si470x: add support for kworld usb radio

This patch add support for new device named KWorld USB FM Radio
SnapMusic Mobile 700 (FM700).
And changes few lines in comments.

Signed-off-by: Alexey Klimov <klimov.linux@gmail.com>
Acked-by: Tobias Lorenz <tobias.lorenz@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>