cmd_jffs2: Fix get_part_sector_size_nor() overflow bug

When a flash partition was positioned at the very top of a 32-bit memory
map (eg located at 0xf8000000 with a size of 0x8000000)
get_part_sector_size_nor() would incorrectly calculate the partition's
ending address to 0x0 due to overflow.  When the overflow occurred
get_part_sector_size_nor() would falsely return a sector size of 0.
A sector size of 0 results in subsequent jffs2 operations failing.

To workaround the overflow subtract 1 from calculated address of
the partition endpoint.

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>