binarybuffer: simplify the prototype of str_to_buf()

With 'radix' always zero and '_detected_radix' always NULL, drop
the two parameters and simplify str_to_buf().

While there:
- drop some redundant assert(),
- drop the re-check for the base prefix,
- simplify str_strip_number_prefix_if_present() and rename it, as
  the prefix MUST be present,
- fix a minor typo,
- update the doxygen description of str_to_buf().

Change-Id: I1abdc8ec0587b23881953d3094101c04d5bb1c58
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8394
Tested-by: jenkins
Reviewed-by: Jan Matyas <jan.matyas@codasip.com>

src/helper/command.c

@@ -1365,7 +1365,7 @@ COMMAND_HELPER(command_parse_str_to_buf, const char *str, void *buf, unsigned in
 	assert(str);
 	assert(buf);
 
-	int ret = str_to_buf(str, buf, buf_len, 0, NULL);
+	int ret = str_to_buf(str, buf, buf_len);
 	if (ret == ERROR_OK)
 		return ret;