vmce: Allow vmce_amd_* functions to handle AMD thresolding MSRs

vmce: Allow vmce_amd_* functions to handle AMD thresolding MSRs

vmce_amd_[rd|wr]msr functions can handle accesses to AMD thresholding
registers. But due to this statement here:
switch ( msr & (MSR_IA32_MC0_CTL | 3) )
we are wrongly masking off top two bits which meant the register
accesses never made it to vmce_amd_* functions.

Corrected this problem by modifying the mask in this patch to allow
AMD thresholding registers to fall to 'default' case which in turn
allows vmce_amd_* functions to handle access to the registers.

While at it, remove some clutter in the vmce_amd* functions. Retained
current policy of returning zero for reads and ignoring writes.

Signed-off-by: Aravind Gopalakrishnan <aravind.gopalakrishnan@amd.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Liu Jinsong <jinsong.liu@intel.com>