libpayload/libc: Tidy utf16le_to_ascii

- Constify the string argument
- Change int to size_t, which is what xmalloc expects

Change-Id: I8b5a13319ded4025f883760f2b6d4d7a9ad9fb8b
Signed-off-by: Jacob Garber <jgarber1@ualberta.ca>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/33793
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>

payloads/libpayload/include/string.h

@@ -73,7 +73,7 @@ char *strerror(int errnum);
  * @defgroup string Unicode functions
  * @{
  */
-char *utf16le_to_ascii(uint16_t *utf16_string, int maxlen);
+char *utf16le_to_ascii(const uint16_t *utf16_string, size_t maxlen);
 /** @} */
 
 /**