powerpc/bpf ppc32: Fix ALU32 BPF_ARSH operation

Correct the destination register used for ALU32 BPF_ARSH operation.

Fixes: 51c66ad8 ("powerpc/bpf: Implement extended BPF on PPC32")
Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/6d24c1f9e79b6f61f5135eaf2ea1e8bcd4dac87b.1633464148.git.naveen.n.rao@linux.vnet.ibm.com