dfu: Avoid declaring unused variables and absent parameters

dfu: Avoid declaring unused variables and absent parameters

The compiler is not happy when neither USB nor TFTP transport for DFU defined:

cmd/dfu.c: In function ‘do_dfu’:
cmd/dfu.c:31:8: warning: unused variable ‘devstring’ [-Wunused-variable]
  char *devstring = argv[3];
        ^~~~~~~~~
cmd/dfu.c:30:8: warning: unused variable ‘interface’ [-Wunused-variable]
    char *interface = argv[2];
          ^~~~~~~~~

Surround those variables by #ifdef expression.

More serious, that comes under same circumstances, is a compilation error due
to absence of macro parameter:

In file included from include/image.h:45,
                 from include/common.h:35,
                 from cmd/dfu.c:13:
include/command.h:207:24: error: expected expression before ‘,’ token
 # define _CMD_HELP(x) x,
                        ^
include/command.h:286:18: note: in expansion of macro ‘_CMD_HELP’
    _cmd, _usage, _CMD_HELP(_help) _CMD_COMPLETE(_comp) }
                  ^~~~~~~~~
include/command.h:290:3: note: in expansion of macro ‘U_BOOT_CMD_MKENT_COMPLETE’
   U_BOOT_CMD_MKENT_COMPLETE(_name, _maxargs, _rep, _cmd, \
   ^~~~~~~~~~~~~~~~~~~~~~~~~
include/command.h:332:2: note: in expansion of macro ‘U_BOOT_CMD_COMPLETE’
  U_BOOT_CMD_COMPLETE(_name, _maxargs, _rep, _cmd, _usage, _help, NULL)
  ^~~~~~~~~~~~~~~~~~~
cmd/dfu.c:70:1: note: in expansion of macro ‘U_BOOT_CMD’
 U_BOOT_CMD(dfu, CONFIG_SYS_MAXARGS, 1, do_dfu,
 ^~~~~~~~~~
make[1]: *** [scripts/Makefile.build:279: cmd/dfu.o] Error 1
make: *** [Makefile:1518: cmd] Error 2

Put empty string unconditionally to have macro parameter present.

Fixes: 0f44d33536a5 ("dfu: Fix up the Kconfig mess")
Cc: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Lukasz Majewski <lukma@denx.de>