x86, MCE, AMD: Make APIC LVT thresholding interrupt optional

commit f227d4306cf30e1d5b6f231e8ef9006c34f3d186 upstream.

Currently, the APIC LVT interrupt for error thresholding is implicitly
enabled. However, there are models in the F15h range which do not enable
it. Make the code machinery which sets up the APIC interrupt support
an optional setting and add an ->interrupt_capable member to the bank
representation mirroring that capability and enable the interrupt offset
programming only if it is true.

Simplify code and fixup comment style while at it.

This patch is for stable kernels v3.0 to v3.2.

Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>
Signed-off-by: Robert Richter <robert.richter@amd.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>