x86: fix warning in arch/x86/kernel/microcode_amd.c

this warning:

  arch/x86/kernel/microcode_amd.c: In function ‘apply_microcode_amd’:
  arch/x86/kernel/microcode_amd.c:163: warning: cast from pointer to integer of different size
  arch/x86/kernel/microcode_amd.c:163: warning: cast from pointer to integer of different size

triggers because we want to pass the address to the microcode MSR,
which is 64-bit even on 32-bit. Cast it explicitly to express this.

Signed-off-by: Ingo Molnar <mingo@elte.hu>