coova-chilli: Fix unwanted startup of disabled instances

coova-chilli: Fix unwanted startup of disabled instances

Code in option_cb was referencing $chilli_inst variable which was
declared as local, thus the instance startup logic in start_chilli was
referencing variable which would always get value of 1, effectively
making `disabled` config option useless.

So I've fixed it with simpler config_get_bool and while at it, I've simplified the
surrounding code little bit as well.

Signed-off-by: Petr Štetiar <redacted>