rule-syntax-check: PROGRAM is not supposed to get value assigned

rule-syntax-check: PROGRAM is not supposed to get value assigned

In udev man page, "PROGRAM" key is part of the keys which are used for
matching purposes so it should only be used with the compare operator "==".

Actually it doesn't really make sense to assign it a value.

udev code allows both "=" and "==" for PROGRAM and both are handled the same
way but for consistencies it's better to have only the compare operator allowed
by the rule syntax checker.

No rules shipped by systemd use PROGRAM key so nothing need to be changed in
our rule files.