cmd: gpt: fix the wrong size parse for the last partition

cmd: gpt: fix the wrong size parse for the last partition

The calculation of "dev_desc->lba - 34  - 1 - offset" is not correct for
size '-', because both fist_usable_lba and last_usable_lba will remain
34 sectors.

We can simply use 0 for size '-' because the part_efi module will decode
the size and auto extend the size to maximum available size.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>