Device Discovery: Check if device is known before trying to connect (DRY)

[CloCkWeRX]

Fixes #968

I'm unclear why the else-if condition exists with basically duplicate logic, so removed the first version in favour of the second

Testing locally by:

• Remove device
• Restart
• Let it be discovered.

Regression test results

• Removed specific device (bfa****gola)
• config/integrations -> Download diagnostics -> confirmed device key not present in the data array.
• Started device discovery (discovered ok)
  

The device + entity are successfully added
(For some reason, I can't control it still, but I assume it is unrelated)

[palfrey]

Applied this manually locally, and it definitely gets rid of the exception, but still wondering about the actual underlying cause given I can't configure the device!

[sanyatuning]

I have the same problem when starting after a power loss.

  File "/config/custom_components/localtuya/__init__.py", line 142, in _device_discovered
    device = hass.data[DOMAIN][TUYA_DEVICES][device_id]
KeyError: 'bf****'

I think this will fix some errors like #1376

@rospogrigio pls review this