[media] dvb: Document FE_SCALE_DECIBEL units consistently

In comments and in the documentation, the units of properties marked
with the FE_SCALE_DECIBEL scale are specified in terms of 1/1000 dB
or 0.0001 dB.  This is inconsistent, however, as 1/1000 is 0.001,
not 0.0001.

Note that the v4l-utils divide the value by 1000 for the signal
strength suggesting that the 1/1000 is correct.

Settle on millidecibels, ie. 1/1000dB or 0.001dB.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>