cmd: ti: ddr3: Fix ecc address calculation

ecc_address_range registers contains the start address and end address
of the DDR address space. But the ddr cmd driver is assuming the register
contains the start address and size of the DDR address space. Because
of this some valid ecc addresses are errored out as invalid address.
Fix this calculation.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>