libata: Avoid overflow in ata_tf_read_block() when tf->hba_lbal > 127

Phillip O'Donnell <phillip.odonnell@gmail.com> pointed out that the same
sign extension bug that was fixed in commit ba14a9c2 ("libata: Avoid
overflow in ata_tf_to_lba48() when tf->hba_lbal > 127") also appears to
exist in ata_tf_read_block().  Fix this by adding a cast to u64.

Signed-off-by: Roland Dreier <rolandd@cisco.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>