bash-completion: use a better definition of __contains_word

- scope the iterator var
- use the correct, quoted, non-expansion prone positional parameter
  notation
- prevent expansion on RHS of comparison
- remove unneeded explicit returns.

This really should be defined only once...
bash-completion: simplify udevadm completion

The AA is unnecessary and only adds needless complexity. Replace it
with a case statement instead of repeatedly calling __contains_word to
overglorify string equalities.