The DCP trusted key type uses the wrong helper function to store
the blob's payload length which can lead to the wrong byte order
being used in case this would ever run on big endian architectures.

Fix by using correct helper function.

Cc: stable@vger.kernel.org # v6.10+
Fixes: 2e8a0f40 ("KEYS: trusted: Introduce NXP DCP-backed trusted keys")
Suggested-by: Richard Weinberger <richard@nod.at>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202405240610.fj53EK0q-lkp@intel.com/Signed-off-by: David Gstir <david@sigma-star.at>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>