usb: gadget: f_uac2: fix return value for UAC2_ATTRIBUTE_STRING store

The configfs store callback should return the number of bytes consumed
not the total number of bytes we actually stored. These could differ if
for example the passed in string had a newline we did not store.

If the returned value does not match the number of bytes written the
writer might assume a failure or keep trying to write the remaining bytes.

For example the following command will hang trying to write the final
newline over and over again (tested on bash 2.05b):

  echo foo > function_name

Fixes: 993a44fa ("usb: gadget: f_uac2: allow changing interface name via configfs")
Cc: stable <stable@kernel.org>
Signed-off-by: Kevin Groeneveld <kgroeneveld@lenbrook.com>
Link: https://lore.kernel.org/r/20241006232637.4267-1-kgroeneveld@lenbrook.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/gadget/function/f_uac2.c

@@ -2061,7 +2061,7 @@ static ssize_t f_uac2_opts_##name##_store(struct config_item *item,	\
 					  const char *page, size_t len)	\
 {									\
 	struct f_uac2_opts *opts = to_f_uac2_opts(item);		\
-	int ret = 0;							\
+	int ret = len;							\
 									\
 	mutex_lock(&opts->lock);					\
 	if (opts->refcnt) {						\
@@ -2072,7 +2072,7 @@ static ssize_t f_uac2_opts_##name##_store(struct config_item *item,	\
 	if (len && page[len - 1] == '\n')				\
 		len--;							\
 									\
-	ret = scnprintf(opts->name, min(sizeof(opts->name), len + 1),	\
+	scnprintf(opts->name, min(sizeof(opts->name), len + 1),		\
 		  "%s", page);						\
 									\
 end:									\