INSN_BITS and comedi_dio_bitfield() changes

I've added a new function comedi_dio_bitfield2() to replace 
comedi_dio_bitfield().  It adds an extra argument that lets you specify a 
"base channel" which corresponds to the least significant bit in your 
bitfields.  The idea is to make comedi_dio_bitfield() useful on dio 
subdevices with more than 32 channels.  It is implemented by passing the 
base channel in the chanspec of the INSN_BITS instruction.  Currently, 
comedi_dio_bitfield2() only works properly for the NI 6509, which has a 96 
channel dio subdevice.

Before I go and add some bitshifts to all the drivers implementing 
insn_bits for digital subdevices, I thought I'd ask if anyone has any 
comments on the change?

-- 
Frank