aoe: suppress compiler warnings

This patch fixes following compiler warnings:

  drivers/block/aoe/aoecmd.c: In function `aoecmd_ata_rw':
  drivers/block/aoe/aoecmd.c:383:17: warning: variable `t' set but not used [-Wunused-but-set-variable]
    struct aoetgt *t;
                   ^
  drivers/block/aoe/aoecmd.c: In function `resend':
  drivers/block/aoe/aoecmd.c:488:21: warning: variable `ah' set but not used [-Wunused-but-set-variable]
    struct aoe_atahdr *ah;
                       ^

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Ed Cashin <ecashin@coraid.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>