staging: comedi: ni_6527: tidy up ni6527_{di, do}_insn_bits()

The digital input and output ports are different. Split the CamelCase
Port_Register() define into separate NI6527_DI_REG() and NI6527_DO_REG()
defines to make this clear.

Tidy up the functions a bit.

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>