staging: comedi: das1800: remove 'qram_len' boardinfo

The "hc" type boards have 64 analog input channels with a 64
entry queue. All the others have 16 channels and a 256 entry
queue.

EXP-1800 expansion boards can be used to increase the number
of analog inputs on the 16 channel boards, 16 channels per
EXP-1800, for a total of 256 channels.

Remove the 'qram_len' member of the boardinfo and use the
'id' member to determine the number of channels.

Add a comment about the number of channels and the unclear
mapping of the channels when EXP-1800 boards are used.

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>