powerpc: fix typo 'CONFIG_PPC_CPU'

commit b69a1da94f3d1589d1942b5d1b384d8cfaac4500 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

	modified:   arch/powerpc/lib/sstep.c
Change-Id: Ib6a218af02c87adf880b6e58decfd120ab40e7d9