[media] cxd2841er: Do some changes at the dvbv5 stats logic

It is a good idea to measure the signal strength while
tuning, as this helps to identify if the antenna is ok.
Also, such measure helps to identify the quality of the
signal.

Do some changes to enable it before signal lock. While
here, optimize the code to only initialize the stats
length once, and make sure that, just after set_frontend,
any reading for the stats that depends on lock to return
FE_SCALE_NOT_AVAILABLE.

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>