hwmon: (lm90) Fix error return value from detect function

lm90_detect_nuvoton() is supposed to return NULL if it can not detect
a chip, or a pointer to the chip name if it does. Under some circumstances
it returns an error pointer instead. Some versions of gcc interpret an
ERR_PTR as region of size 0 and generate an error message.

  In function ‘__fortify_strlen’,
      inlined from ‘strlcpy’ at ./include/linux/fortify-string.h:159:10,
      inlined from ‘lm90_detect’ at drivers/hwmon/lm90.c:2550:2:
  ./include/linux/fortify-string.h:50:33: error:
      ‘__builtin_strlen’ reading 1 or more bytes from a region of size 0
     50 | #define __underlying_strlen     __builtin_strlen
        |                                 ^
  ./include/linux/fortify-string.h:141:24: note:
      in expansion of macro ‘__underlying_strlen’
    141 |                 return __underlying_strlen(p);
        |                        ^~~~~~~~~~~~~~~~~~~

Returning NULL instead of ERR_PTR() fixes the problem.

Fixes: c7cebce9 ("hwmon: (lm90) Rework detect function")
Reported-by: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Kees Cook <keescook@chromium.org>
Tested-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>