staging: comedi: ni_labpc: fix labpc_calib_insn_write()

The comedi core expects the (*insn_write) operations to write insn->n
values and return the number of values actually wrote.

Make this function work like the core expects.

As Ian Abbott pointed out for the eeprom writes, we really only need
to write the last value to the caldac. The preceding data would be
overwritten anyway.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>