boot_serial: fix misuse of 'matched' param from zcbor_map_decode_bulk()

The 'matched' param in zcbor_map_decode_bulk() function is 'pointer to
the counter of matched keys', not length of payload buffer.

Fixes: fac2cab ("boot_serial: Add image state set/get")
Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>

boot/boot_serial/src/boot_serial.c

@@ -440,7 +440,7 @@ bs_set(char *buf, int len)
     ok = zcbor_map_decode_bulk(zsd, image_set_state_decode, ARRAY_SIZE(image_set_state_decode),
                                &decoded) == 0;
 
-    if (!ok || len != decoded) {
+    if (!ok) {
         rc = MGMT_ERR_EINVAL;
         goto out;
     }