disk: Replace transform_sector() function with grub_disk_to_native_sector()

disk: Replace transform_sector() function with grub_disk_to_native_sector()

The transform_sector() function is not very clear in what it's doing
and confusing. The GRUB already has a function which is doing the same
thing in a very self explanatory way, i.e., grub_disk_to_native_sector().
So, it's much better to use self explanatory one than transform_sector().

Signed-off-by: Mukesh Kumar Chaurasiya <mchauras@linux.vnet.ibm.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>