[media] cxd2841er: Fix signal strengh for DVB-T/T2 and show it in dBm

The signal strength value is reversed: the bigger the number,
the weaker is the signal.

Fix the logic and present it in dBm. Please notice that the
dBm measure is actually an estimation, as the ratio is not
fully linear. It also varies with the frequency.

Yet, the estimation should be good enough for programs like
Kaffeine to indicate when the signal is good or bad.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Acked-by: Abylay Ospan <aospan@netup.ru>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>