util/bash-completion: Fix SC2155 shellcheck warning

util/bash-completion: Fix SC2155 shellcheck warning

SC2155 (warning): Declare and assign separately to avoid masking return values.

The exit status of the command is overridden by the exit status of the
creation of the local variable.

In grub-completion.bash.in line 115:
    local config_file=$(__grub_dir)/grub.cfg
          ^---------^ SC2155 (warning)

In grub-completion.bash.in line 126:
    local grub_dir=$(__grub_dir)
          ^------^ SC2155 (warning)

More: https://github.com/koalaman/shellcheck/wiki/SC2155

Signed-off-by: t.feng <fengtao40@huawei.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>