staging: comedi: adv_pci_dio: refactor 'io_access' boardinfo

The boards supported by this driver either use 8-bit or 16-bit I/O. The
'io_access' member of the boardinfo is used by the (*auto_attach) to
determine which (*insn_bits) function to use.

Simplify the boardinfo a bit by refactoring the 'io_access' member into
a bit-field flag 'is_16bit'. Use the new flag and remove the switch ()
code in the (*auto_attach).

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>