net: usb: cdc_ether: don't spew notifications

The usbnet_link_change function is not called, if the link has not changed.

...
[16913.807393][ 3] cdc_ether 1-2:2.0 enx00e0995fd1ac: kevent 12 may have been dropped
[16913.822266][ 2] cdc_ether 1-2:2.0 enx00e0995fd1ac: kevent 12 may have been dropped
[16913.826296][ 2] cdc_ether 1-2:2.0 enx00e0995fd1ac: kevent 11 may have been dropped
...

kevent 11 is scheduled too frequently and may affect other event schedules.

Signed-off-by: zhangxiangqian <zhangxiangqian@kylinos.cn>
Acked-by: Oliver Neukum <oneukum@suse.com>
Link: https://patch.msgid.link/1723109985-11996-1-git-send-email-zhangxiangqian@kylinos.cn


Signed-off-by: Jakub Kicinski <kuba@kernel.org>