west_commands: sign: imgtool: fix slot size readout

This fixes wrong slot selection when reading partition size. According
to the comment in code, 'slot1_partition' size should be preferred:

  [...] slot1_partition size is used, when available, because in
  swap-move mode it can be one sector smaller. When not available,
  fallback to slot0_partition (single slot dfu). [...]

This fixes a typo in the if statement which currently always results in
use of 'slot0_partition' size.

Fixes: 86c4b4ca ("west/sign: Move from using partition label property")
Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>