staging: comedi: adv_pci1710: change boardinfo 'n_counter' to 'has_counter'

The 'n_counter' member of the boardinfo is actually a flag indicating that the
board exposes a single channel counter subdevice.

For aesthetics, change the 'n_counter' member to a bit-field flag 'has_counter'
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.

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>