x86/microcode/AMD: Cleanup verify_patch_size() more

Rename the variable which contains the patch size read out from the
section header to sh_psize for better differentiation of all the "sizes"
in that function.

Also, improve the comment above it.

No functional changes.

Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: x86@kernel.org
Link: https://lkml.kernel.org/r/20181107170218.7596-6-bp@alien8.de