V4L/DVB (3250): tea5767: move signal strength level to the 0-65535 range

- Move signal strength level to the 0-65535 range as per V4L2 spec.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@brturbo.com.br>