cryptodisk: Add macros GRUB_TYPE_U_MAX/MIN(type) to replace literals

Add GRUB_TYPE_U_MAX/MIN(type) macros to get the max/min values for an
unsigned number with size of type.

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