Now when I run dmesg I get Bad Firmware Length instead of the dvb-tuner-si2158-a20-01.fw line…
Did you check the checksums after downloading?
Also there are some DVB scanning message. Is TVHeadend seeing the tuner now?
No, I haven’t ran the checksums. What is the command for that? 
TVheadend had the adapter in the TV adapter list before but didn’t work. Hence when I ran dmesg in the beginning it told about missing firmware.
Do I run this command:
sudo apt-get install debsums
?
If they are deb sums yes, otherwise md5sum
I have run debsums and I still have the messages in red saying:
Firmware download failed -121
Bad firmware length
When I run dmesg there are also red messages further up but it moves to quickly to see what they say…
Logs are here: http://paste.osmc.io/tidulucuti
So have you compared the check sums and ensured your downloaded files are clean?
Your errors still indicate problems with the files. Not much I can help here.
I ran this…
Then what? I still had the same problems. I.e. bad firmware length. 
Appreciate your patience.
Sudo debsums -ca?