lib/vsprintf.c: Amend static asserts for format specifier flags

There are couple of improvements to static asserts against
the format specifier flags:

- new static assert for SIGN
- fix static assert for SMALL

SMALL is not equal to ASCII code of white space, it equals to
the bit difference between capital and small letters (however
the value is the same, semantically expression means different
things).

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20211026140356.45610-1-andriy.shevchenko@linux.intel.com