powerpc: fix typo 'CONFIG_PPC_CPU'

Commit cd64d1697cf0 ("powerpc: mtmsrd not defined") added a check for
CONFIG_PPC_CPU were a check for CONFIG_PPC_FPU was clearly intended.

Fixes: cd64d1697cf0 ("powerpc: mtmsrd not defined")
Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/lib/sstep.c

@@ -1470,7 +1470,7 @@ int __kprobes emulate_step(struct pt_regs *regs, unsigned int instr)
 				regs->gpr[rd] = byterev_4(val);
 			goto ldst_done;
 
-#ifdef CONFIG_PPC_CPU
+#ifdef CONFIG_PPC_FPU
 		case 535:	/* lfsx */
 		case 567:	/* lfsux */
 			if (!(regs->msr & MSR_FP))