Add support for new Kobo Aura H2O and firmware 3.8.0

Kobo have announced a new device. It uses the same firmware as their
other devices. This adds the device id and name.

There is also a new firmware version released. This is version 3.8.0,
but I am allowing 3.8.1. There are no changes needed to support the
firmware.

Fixes #1362435 [Add support for new Kobo Aura H2O and firmware 3.8.0](https://bugs.launchpad.net/calibre/+bug/1362435)
Fix a typo preventing calibre from recognizing the new Kobo Aura H20

I made a typo when adding the device id for the new Kobo Aura H20.
Fixes #1375748 [Correct device id for Kobo Aura H20](https://bugs.launchpad.net/calibre/+bug/1375748)
Kobo driver: Update to support new firmware from Kobo. Fixes #1382051 [Kobo: Skip modifying epub if no valid CSS rules and bump firmware version](https://bugs.launchpad.net/calibre/+bug/1382051)

Skip modifying epub if no valid CSS rules and bump firmware version

While looking at a problem with the stylesheet on Kobo devices recently,
I realised that if the modify CSS option was set and the was no file, it
still modified the stylesheet. Now, will not do this if there are no
valid rules in the kobo_extra.css file.

Also bumping firmware version to 3.11.1. Kobo are about to release an
update. They big jump on version number is to match the desktop
application version.