ls: prevent double open

ls: prevent double open

Prevent a double open.  This can cause problems with some ieee1275
devices, causing the system to hang.  The double open can occur
as follows:

grub_ls_list_files (char *dirname, int longlist, int all, int human)
       dev = grub_device_open (device_name);
       dev remains open while:
       grub_normal_print_device_info (device_name);
                dev = grub_device_open (name);

Signed-off-by: Eric Snowberg <eric.snowberg@oracle.com>
Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>