staging: axis-fifo: Fix kernel-doc return value warnings

The kernel-doc comments for axis_fifo_read() and axis_fifo_write() describe their return values as free text, which kernel-doc does not recognize as a return section:

  $ scripts/kernel-doc -Wall -none drivers/staging/axis-fifo/axis-fifo.c
  Warning: drivers/staging/axis-fifo/axis-fifo.c:121 No description
  found for return value of 'axis_fifo_read'
  Warning: drivers/staging/axis-fifo/axis-fifo.c:214 No description
  found for return value of 'axis_fifo_write'

These warnings only show up in a direct kernel-doc invocation or a W=2 build (-Wall is added to kernel-doc only when KBUILD_EXTRA_WARN contains 2), which is why W=1 builds appear clean.

Convert the trailing "Returns ..." sentences into Return: sections so kernel-doc recognizes the existing return value documentation. No functional change.

---
 drivers/staging/axis-fifo/axis-fifo.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

index 3aa2aa870ea9..7311e06312b0 100644
@@ -114,7 +114,7 @@ static void reset_ip_core(struct axis_fifo *fifo)
  * operations must be executed atomically, in order and one after the other
  * without missing any.
  *
- * Returns the number of bytes read from the device or negative error code
+ * Return: The number of bytes read from the device or negative error code
  *	on failure.
  */
 static ssize_t axis_fifo_read(struct file *f, char __user *buf,

@@ -207,7 +207,7 @@ static ssize_t axis_fifo_read(struct file *f, char __user *buf,
  * we need to lock before checking if the device has available space to avoid
  * any concurrency issue.
  *
- * Returns the number of bytes written to the device or negative error code
+ * Return: The number of bytes written to the device or negative error code
  *	on failure.
  */
 static ssize_t axis_fifo_write(struct file *f, const char __user *buf,

base-commit: f4fb100039e96211609dfc44fb24b9e4a8a0f2f9

2.43.0