lib/vsprintf: Use isodigit() for the octal number check

Use isodigit() to test the octal number instead of homegrown approach.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20230327142721.48378-1-andriy.shevchenko@linux.intel.com