drm/vmwgfx/vmwgfx_ioctl: Provide missing '@' sign required by kernel-doc

Fixes the following W=1 kernel build warning(s):

 In file included from drivers/gpu/drm/vmwgfx/vmwgfx_ioctl.c:30:
 drivers/gpu/drm/vmwgfx/vmwgfx_ioctl.c:448: warning: Function parameter or member 'offset' not described in 'vmw_fops_read'

Cc: VMware Graphics <linux-graphics-maintainer@vmware.com>
Cc: Roland Scheidegger <sroland@vmware.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Zack Rusin <zackr@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210115181313.3431493-10-lee.jones@linaro.org

drivers/gpu/drm/vmwgfx/vmwgfx_ioctl.c

@@ -437,7 +437,7 @@ __poll_t vmw_fops_poll(struct file *filp, struct poll_table_struct *wait)
  * @filp: See the linux fops read documentation.
  * @buffer: See the linux fops read documentation.
  * @count: See the linux fops read documentation.
- * offset: See the linux fops read documentation.
+ * @offset: See the linux fops read documentation.
  *
  * Wrapper around the drm_read function that makes sure the device is
  * processing the fifo if drm_read decides to wait.