libata: make ata_tf_to_lba[48]() generic

ata_tf_to_lba[48]() currently return LBA in tf + 1 for
ata_read_native_max_address().  Make them return LBA and make it
global so that it can be used to read LBA off TF for other purposes.
ata_read_native_max_address() now adds 1 itself.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>