x86/microcode/AMD: Make stub function static inline

When building with C=1 W=1 (and when CONFIG_MICROCODE_AMD=n, as Luc Van
Oostenryck correctly points out) both sparse and gcc complain:

  CHECK   arch/x86/kernel/cpu/microcode/core.c
  ./arch/x86/include/asm/microcode_amd.h:56:6: warning: symbol \
	  'reload_ucode_amd' was not declared. Should it be static?
    CC      arch/x86/kernel/cpu/microcode/core.o
  In file included from arch/x86/kernel/cpu/microcode/core.c:36:
  ./arch/x86/include/asm/microcode_amd.h:56:6: warning: no previous \
	  prototype for 'reload_ucode_amd' [-Wmissing-prototypes]
     56 | void reload_ucode_amd(void) {}
        |      ^~~~~~~~~~~~~~~~

And they're right - that function can be a static inline like its
brethren.

Signed-off-by: Valdis Klētnieks <valdis.kletnieks@vt.edu>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Cc: x86@kernel.org
Link: https://lkml.kernel.org/r/52170.1575603873@turing-police