x86/microcode/AMD: Fix a -Wsometimes-uninitialized clang false positive

x86/microcode/AMD: Fix a -Wsometimes-uninitialized clang false positive

Initialize equiv_id in order to shut up:

  arch/x86/kernel/cpu/microcode/amd.c:714:6: warning: variable 'equiv_id' is \
  used uninitialized whenever 'if' condition is false [-Wsometimes-uninitialized]
        if (x86_family(bsp_cpuid_1_eax) < 0x17) {
            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

because clang doesn't do interprocedural analysis for warnings to see
that this variable won't be used uninitialized.

Fixes: 94838d230a6c ("x86/microcode/AMD: Use the family,model,stepping encoded in the patch ID")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202407291815.gJBST0P3-lkp@intel.com/
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>