aoe: remove unecessary wrapper function

We can just use skb_mac_header now, and we don't need a wrapper function to
perform the cast.  Instead of requiring the reader to check aoe.h to look
up what an aoe_hdr function does, I'd rather do without it.

Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>