NFS: Squelch compiler warning in decode_getdeviceinfo()

Clean up.

.../linux/nfs-2.6/fs/nfs/nfs4xdr.c: In function ‘decode_getdeviceinfo’:
.../linux/nfs-2.6/fs/nfs/nfs4xdr.c:5008: warning: comparison between signed and unsigned integer expressions

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Tested-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>