boot: boot_serial: fix usage of zcbor_new_encode_state API

Fix usage of zcbor_new_encode_state API, to correctly pass the payload
length. The previous usage was passing a pointer to the end of the
payload, which resulted in the ZCBOR structure being initialized with
an invalid `payload_end` field. On some platforms, this breaks MCUBoot
serial recovery, as the ZCBOR structures required to send response data
are invalid and can no longer be populated with response data.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>