staging: comedi: ni_660x: cleanup ni_660x_dio_insn_config()

This function handles all the insn_config instructions for the
digital i/o subdevice. These functions are supposed to return
the number of instruction parameters used (insn->n) or an -errno.
Fix the switch() so that the correct result is returned for all
INSN_CONFIG_* cases.

To clarify the code, add a local variable for the 'bit' used with
the instructions used to configure and query the input/output setting
of a channel.

For aesthetic reasons, add a whitespace between each case to improve
readability.

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>