cx82310_eth: re-enable ethernet mode after router reboot

[ Upstream commit ca139d76b0d9e59d18f2d2ec8f0d81b82acd6808 ]

When the router is rebooted without a power cycle, the USB device
remains connected but its configuration is reset. This results in
a non-working ethernet connection with messages like this in syslog:
	usb 2-2: RX packet too long: 65535 B

Re-enable ethernet mode when receiving a packet with invalid size of
0xffff.

Signed-off-by: Ondrej Zary <linux@zary.sk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Stable-dep-of: bab8eb0dd4cb ("usbnet: modern method to get random MAC")
Signed-off-by: Sasha Levin <sashal@kernel.org>