staging: comedi: fix return value for insn_bits functions

The comedi_subdevice 'insn_bits' functions return the number of data
elements used to perform the command. Most of the insn_bits functions
return an open coded '2' to indicate this. The same value is available
as 'insn->n'. Return that instead to better indicate what the return
means.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Cc: Frank Mori Hess <fmhess@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>