net: lan78xx: fix transfer-buffer memory leak

[ Upstream commit 63634aa679ba8b5e306ad0727120309ae6ba8a8e ]

The interrupt URB transfer-buffer was never freed on disconnect or after
probe errors.

Fixes: 55d7de9d ("Microchip's LAN7800 family USB 2/3 to 10/100/1000 Ethernet device driver")
Cc: Woojung.Huh@microchip.com <Woojung.Huh@microchip.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>