m25p80: fix s->blk usage before assignment

Delay the call to blk_blockalign() until s->blk has been assigned.

This never caused a crash because blk_blockalign(NULL, size) defaults to
4096 alignment but it's technically incorrect.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1429091024-25098-1-git-send-email-stefanha@redhat.com
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
