powerpc/module64: Fix comment in R_PPC64_ENTRY handling

The comment here is wrong, the addi reads from r2 not r12. The code is
correct, 0x38420000 = addi r2,r2,0.

Fixes: a61674bd ("powerpc/module: Handle R_PPC64_ENTRY relocations")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>