can: gw: use kmem_cache_free() instead of kfree()

Memory allocated by kmem_cache_alloc() should be freed using
kmem_cache_free(), not kfree().

Cc: linux-stable <stable@vger.kernel.org> # >= v3.2
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>