can: sja1000: fix bug using library functions for skb allocation

Commit 7b6856a0 "can: provide library functions for skb allocation"
did not properly remove two lines of the SJA1000 driver resulting in
a 'skb_over_panic' when calling skb_put, as reported by Kurt.

Signed-off-by: Kurt Van Dijck <kurt.van.dijck@eia.be>
Signed-off-by: Wolfgang Grandegger <wg@grandegger.com>
Signed-off-by: David S. Miller <davem@davemloft.net>