staging: comedi: ni_usb6501: add counter subdevice
staging: comedi: ni_usb6501: add counter subdevice

Add counter support for NI USB-6501.

The following functions are introduced:
- ni6501_counter_command()
- ni6501_cnt_insn_config()
- ni6501_cnt_insn_read()
- ni6501_cnt_insn_write()

Signed-off-by: Luca Ellero <c609f7af1be5264f17b152fbd89761fd6729a1f3@brickedbrain.com>
Reviewed-by: H Hartley Sweeten <382ff55d8e07d1082d179e669636cd1552da4f36@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <4645f7897fd33786a2ee1264d590b3c400559d85@linuxfoundation.org>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs