libata: fix ata_id_logical_per_physical_sectors

The value we get from the low byte of the ATA_ID_SECTOR_SIZE word is not not
a plain multiple, but the log of it, so fix the helper to give the correct
answer.  Without this we'll get an incorrect minimal I/O size in the block
limits VPD page for 4k sector drives.

Also change the return value of ata_id_logical_per_physical_sectors to u16
for the unlikely case of very large logical sectors.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>