bash-completion: fix shellcheck SC2268

        if [[ "x$1" = "xcd" ]]; then
              ^---^ SC2268: Avoid x-prefix in comparisons as it no longer serves a purpose.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
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