staging: comedi: ni_labpc: fix labpc_eeprom_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, we really only need to write the last value
to the eeprom. The preceding data would be overwritten anyway.

Also, remove the noise about invalid channels.

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>