dracut-module-setup.sh: fix a ambiguous variable reference
    
    upstream: fedora
    resolves: bz2003832
    conflict: none
    
    commit dfe7555323e22d3f23621fcdae11b2c467754175
    Author: Kairui Song <kasong@redhat.com>
    Date:   Wed Aug 4 15:51:34 2021 +0800
    
        dracut-module-setup.sh: fix a ambiguous variable reference
    
        Wrap the variable with {...}, else it may get interpreted as array due
        to the '[' char next to it.
    
        Signed-off-by: Kairui Song <kasong@redhat.com>
        Acked-by: Philipp Rudo <prudo@redhat.com>
    
    Signed-off-by: Tao Liu <ltao@redhat.com>