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>

boot/boot_serial/src/boot_serial.c

@@ -136,7 +136,7 @@ static zcbor_state_t cbor_state[2];
 void reset_cbor_state(void)
 {
     zcbor_new_encode_state(cbor_state, 2, (uint8_t *)bs_obuf,
-        (size_t)bs_obuf + sizeof(bs_obuf), 0);
+        sizeof(bs_obuf), 0);
 }
 
 /**