staging: comedi: ni_labpc_common: Use insn->n in AO insn_write handler

The `insn_write` handler for the AO subdevice (`labpc_ao_insn_write()`)
currently ignores `insn->n` (the number of samples to write) and assumes
a single sample is to be written.  But `insn->n` could be 0, meaning no
samples should be written, in which case `data[0]` is invalid.

Follow the usual Comedi guidelines and change `labpc_ao_insn_write()` to
write the specified number of samples.  This fixes the assumption that
`data[0]` is valid.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>