staging: comedi: ke_counter: fix ke_counter_insn_write()

The comedi core expects the (*insn_write) functions to write insn->n
values. Fix this function to work like the core expects.

The counters are actually 25-bit (24-bits + 1 sign bit). The comedi core
validates that all the data is in range before calling the (*insn_write),
fix the subdevice maxdata so that the sign bit can actually be changed.

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>