staging: comedi: ke_counter: add ability to select counter clock source

Add an (*insn_config) to the counter subdevice to allow the user to select
the clock source for the counters using the INSN_CONFIG_SET_CLOCK_SRC
instruction. The current selection can be queried with the instruction
INSN_CONFIG_GET_CLOCK_SRC.

Also, handle the INSN_CONFIG_RESET instruction to reset all the counters.

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