[SCSI] imm: fix check-after-use

The Coverity checker spotted that we have already oops'ed if "cmd"
was NULL.

Since "cmd" being NULL doesn't seem to be possible at this point this
patch removes the NULL check.

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>