Staging: comedi: s526: Fix number of channels on DIO subdevice

Correct operation of INSN_CONFIG_DIO_INPUT and INSN_CONFIG_DIO_OUTPUT
and support INSN_CONFIG_DIO_QUERY.  Thanks to Alessio Margan for some
testing.

Signed-off-by: Frank Mori Hess <fmhess@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>