x86, microcode, AMD: Fix section header size check

The ucode size check has to take the section header size into account
too when sanity checking the section length. Shorten and clarify define
names, while at it.

Caught-by: Ben Hutchings <ben@decadent.org.uk>
Link: http://lkml.kernel.org/r/1302752223.5282.674.camel@localhost


Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>