aoe: convert emsgs_sema into a completion

ATA over Ethernet: The semaphore emsgs_sema is used for signalling an
event, convert it in a completion.

Signed-off-by: Matthias Kaehlcke <matthias@kaehlcke.net>
Cc: "Ed L. Cashin" <ecashin@coraid.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>