Fix zsh completion for "localectl set-locale"

Fix zsh completion for "localectl set-locale"

When running:

    $ localectl set-locale LC_MESSAGES=<TAB>

One is prompted with a list of locale fields instead of the list of
valid locales. This is because by calling "compset -P1 '*='", we modify
the $PREFIX special parameter before testing whether it contains an
equal sign. Therefore

    if [[ -prefix 1 *\= ]]

is always false, and we always suggest a list of locale fields to the
user.

Fixes: #27955