x86/ucode: fix for AMD Fam15 CPUs

Remove hardcoded maximum size a microcode patch can have. This is
dynamic now.

The microcode patch for family15h can be larger than 2048 bytes and
gets silently truncated.

Signed-off-by: Christoph Egger <Christoph.Egger@amd.com>
Backport from xen-unstable changeset 24411:ca5f588bd203 to Xen 4.1


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