drivers/net/ethernet/tile: fix netdev_alloc_skb() bombing

Commit dae2e9f4 changed dev_alloc_skb()
to netdev_alloc_skb(), adding a dev pointer, but erroneously used "->"
instead of "." for a struct member when accessing the dev pointer.
This change fixes the build breakage.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>