aoe: mac_addr: avoid 64-bit arch compiler warnings

By returning unsigned long long, mac_addr does not generate compiler warnings
on 64-bit architectures.

Signed-off-by: Ed L. Cashin <ecashin@coraid.com>
Cc: Greg KH <greg@kroah.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>