net: usb: enable the work after stop usbnet by ip down/up

Oleksij reported that:
The smsc95xx driver fails after one down/up cycle, like this:
 $ nmcli device set enu1u1 managed no
 $ p a a 10.10.10.1/24 dev enu1u1
 $ ping -c 4 10.10.10.3
 $ ip l s dev enu1u1 down
 $ ip l s dev enu1u1 up
 $ ping -c 4 10.10.10.3
The second ping does not reach the host. Networking also fails on other interfaces.

Enable the work by replacing the disable_work_sync() with cancel_work_sync().

[Jun Miao: completely write the commit changelog]

Fixes: 2c04d279 ("net: usb: Convert tasklet API to new bottom half workqueue mechanism")
Reported-by: Oleksij Rempel <o.rempel@pengutronix.de>
Tested-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Zqiang <qiang.zhang@linux.dev>
Signed-off-by: Jun Miao <jun.miao@intel.com>
Link: https://patch.msgid.link/20250708081653.307815-1-jun.miao@intel.com


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