fwu: fix fwu_get_image_index interface

The capsule update uses the DFU framework for updating
storage. fwu_get_image_index() currently returns the
image_index calculated by (dfu_alt_num + 1), but this is
different from the image_index in UEFI terminology.

Since capsule update implementation calls dfu_write_by_alt
function, it is better that FWU returns the dfu_alt_num.

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>