selftests/powerpc: ppc_asm.h: typo in the header guard

The guard macro __PPC_ASM_H in the header ppc_asm.h
doesn't match the #ifndef macro _PPC_ASM_H. The patch
makes them the same.

Signed-off-by: Denis Efremov <efremov@linux.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>