staging: comedi: adv_pci1710: change boardinfo 'n_aochan' to 'has_ao'

Some of the boards supported by this driver have analog outputs. The pci1720
board has 4 analog output channels and the others have 2.

For aesthetics, change the 'n_aochan' member to a bit-field flag 'has_ao' and
refactor the board attach accordingly. Remove the unnecessary initialization
of the subdevice 'len_chanlist'. That member is only used by subdevices that
support async commands.

Also, remove the unnecessary 'number of channels' check in pci171x_reset().
The boards that used this function, and have analog outputs, always have 2
channels.

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>