usb: gadget: f_uac2: fix non-newline-terminated function name

Most writes to configfs handle an optional newline, but do not require
it.  By using the number of bytes written as the limit for scnprintf()
it is guaranteed that the final character in the buffer will be
overwritten.

This is expected if it is a newline but is undesirable when a string is
written "as-is" (as libusbgx does, for example).

Update the store function to strip an optional newline, matching the
behaviour of usb_string_copy().

Signed-off-by: John Keeping <jkeeping@inmusicbrands.com>
Link: https://lore.kernel.org/r/20240708142553.3995022-1-jkeeping@inmusicbrands.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: johnkeeping

drivers/usb/gadget/function/f_uac2.c

@@ -2063,7 +2063,10 @@ static ssize_t f_uac2_opts_##name##_store(struct config_item *item,	\
 		goto end;						\
 	}								\
 									\
-	ret = scnprintf(opts->name, min(sizeof(opts->name), len),	\
+	if (len && page[len - 1] == '\n')				\
+		len--;							\
+									\
+	ret = scnprintf(opts->name, min(sizeof(opts->name), len + 1),	\
 			"%s", page);					\
 									\
 end:									\