sandbox: spi: sandbox_sf_state_name() is required

sandbox: spi: sandbox_sf_state_name() is required

Compiling drivers/mtd/spi/sandbox.c fails when compiled with
CONFIG_LOG=n:

In file included from include/common.h:20,
                 from drivers/mtd/spi/sandbox.c:13:
drivers/mtd/spi/sandbox.c:295:15: error: format ‘%s’ expects argument of
type ‘char *’, but argument 7 has type ‘int’ [-Werror=format=]
  295 |   log_content(" cmd: transition to %s state\n",
      |               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
include/linux/printk.h:37:21: note: in definition of macro ‘pr_fmt’
   37 | #define pr_fmt(fmt) fmt
      |                     ^~~
include/log.h:128:30: note: in expansion of macro ‘log_nop’
  128 | #define log_content(_fmt...) log_nop(LOG_CATEGORY, \
      |                              ^~~~~~~
drivers/mtd/spi/sandbox.c:295:3: note: in expansion of macro
‘log_content’
  295 |   log_content(" cmd: transition to %s state\n",
      |   ^~~~~~~~~~~
drivers/mtd/spi/sandbox.c:295:37: note: format string is defined here
  295 |   log_content(" cmd: transition to %s state\n",
      |                                    ~^
      |                                     |
      |                                     char *
      |                                    %d

Supply function sandbox_sf_state_name() independent of CONFIG_LOG.

Fixes: 7fc08238f396 ("sandbox: spi: Add more logging")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>