m68k: kill skb_copy_from_linear_data compiler warnings

The recent conversion from `memcpy' to `skb_copy_from_linear_data' removed a
few casts, which were needed to silence compiler warnings. Re-add them.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>