firmware: imx: scu: Fix corruption of header

The header of the message to send can be changed if the
response is longer than the request:
 - 1st word, the header is sent
 - the remaining words of the message are sent
 - the response is received asynchronously during the
   execution of the loop, changing the size field in
   the header
 - the for loop test the termination condition using
   the corrupted header

It is the case for the API build_info which has just a
header as request but 3 words in response.

This issue is fixed storing the header locally instead of
using a pointer on it.

Fixes: edbee095 (firmware: imx: add SCU firmware driver support)

Signed-off-by: Franck LENORMAND <franck.lenormand@nxp.com>
Reviewed-by: Leonard Crestez <leonard.crestez@nxp.com>
Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Cc: stable@vger.kernel.org
Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>