iwlwifi: fw: harden page loading code

The page loading code trusts the data provided in the firmware images
a bit too much and may cause a buffer overflow or copy unknown data if
the block sizes don't match what we expect.

To prevent potential problems, harden the code by checking if the
sizes we are copying are what we expect.

Cc: stable@vger.kernel.org
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>