[ATA] Increase lba48 max-sectors from 200 to 256.

Also, moved ATA_MAX_SECTORS and ATA_MAX_SECTORS_LBA48 from
linux/libata.h to linux/ata.h, now that they truly reflect the standard
(well... mostly; note TODO comment).

This changes the performance profile (and potential bug profile)
for a bunch of drivers, so be wary.