x86/lib/msr: Clean up kernel-doc notation

Convert x86/lib/msr.c comments to kernel-doc notation to
eliminate kernel-doc warnings:

  msr.c:30: warning: This comment starts with '/**', but isn't \
  a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
  ...

Fixes: 22085a66 ("x86: Add another set of MSR accessor functions")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/oe-kbuild-all/202304120048.v4uqUq9Q-lkp@intel.com/