x86/amd_nb: Enhance SMN access error checking

AMD Zen-based systems use a System Management Network (SMN) that
provides access to implementation-specific registers.

SMN accesses are done indirectly through an index/data pair in PCI
config space. The accesses can fail for a variety of reasons.

Include code comments to describe some possible scenarios.

Require error checking for callers of amd_smn_read() and amd_smn_write().
This is needed because many error conditions cannot be checked by these
functions.

  [ bp: Touchup comment. ]

Signed-off-by: Yazen Ghannam <[email protected]>
Signed-off-by: Borislav Petkov (AMD) <[email protected]>
Reviewed-by: Mario Limonciello <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

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