disk: Move hardcoded max disk size literal to a GRUB_DISK_MAX_SECTORS in disk.h

disk: Move hardcoded max disk size literal to a GRUB_DISK_MAX_SECTORS in disk.h

There is a hardcoded maximum disk size that can be read or written from,
currently set at 1 EiB in grub_disk_adjust_range(). Move the literal into a
macro in disk.h, so our assumptions are more visible. This hard coded limit
does not prevent using larger disks, just GRUB won't read/write past the
limit. The comment accompanying this restriction didn't quite make sense to
me, so its been modified too.

Signed-off-by: Glenn Washburn <development@efficientek.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>