powernv/pci: Allow any write trigger the diag dump

Make the dump trigger off any input rather than just '1'. This allows you
to write "echo 1> dump_diag_data" and it'll do what you want rather than
erroring out pointlessly.

Signed-off-by: Oliver O'Halloran <oohall@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190912052945.12589-2-oohall@gmail.com