staging: comedi: ke_counter: fix ke_counter_insn_read()

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

The counters are actually 25-bit (24-bits + 1 sign bit). Instead of munging
the data when the sign bit is set just return the full 25-bit value.

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>