nvme: Fix unintentional integer overflow (OVERFLOW_BEFORE_WIDEN)

The shift operation on nlb gives a 32 bit result if no type cast is
applied. This bug was reported by Coverity.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Message-id: 1426348844-8793-1-git-send-email-sw@weilnetz.de
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>