powerpc/ppc-opcode: Fix PPC_RAW_TW()

PPC_RAW_TW() is erroneously defined with base code 0x7f000008
instead of 0x7c000008.

That's invisible because its only user is PPC_RAW_TRAP() which is
0x7fe00008, but fix it anyway to avoid any risk of future bug.

Fixes: d00d762d ("powerpc/ppc-opcode: Define and use PPC_RAW_TRAP() and PPC_RAW_TW()")
Reported-by: Naveen N. Rao <naveen.n.rao@linux.ibm.com>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/eca9251f1e1f82c4c46ec6380ddb28356ab3fdfe.1659527244.git.christophe.leroy@csgroup.eu