powerpc: Fix typo "CONFIG_ICSWX_PID"

Untested. As this typo was introduced in v3.3, with commit
9d670280

 ("powerpc: Split ICSWX ACOP and
PID processing"), which actually added PPC_ICSWX_PID, this surely needs
testing.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>