powerpc: Remove stale calc_vm_prot_bits() comment

This comment is wrong, we wouldn't use calc_vm_prot_bits() here
because we are being called by calc_vm_prot_bits() to modify its
behaviour.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200703011958.1166620-2-npiggin@gmail.com