x86/microcode/AMD: Fix container size's type

Make it size_t everywhere as this is what we get from cpio.

 [ bp: Fix a smatch warning. ]

Originally-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name>
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20181107170218.7596-13-bp@alien8.de