[media] em28xx-dvb: enable LNA for cxd2820r in DVB-T mode

Enable the LNA amplifier also for DVB-T (like for DVB-T2 and DVB-C);
this greatly improves reception of weak signals without affecting the reception
of the strong ones.

Experimental data (collected with the mipsel STB) on the weakest frequencies
available in my area:

LNA OFF:

MUX          level   BER     picture

RAI mux 4    72%     32000   corrupted
TIMB 2       75%     14      OK
TVA Vicenza  68%     32000   corrupted
RAI mux 2    78%     14      OK

LNA ON:

MUX          level   BER     picture

RAI mux 4    73%     1500    OK
TIMB 2       76%     0       OK
TVA Vicenza  69%     0       OK
RAI mux 2    79%     0       OK

Moreover, with LNA enabled, the PCTV 290e was able to pick up 2 new frequencies
matching the integrated tuner of my Panasonic G20 TV, which is really good.

Signed-off-by: Gianluca Gennari <gennarone@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>