staging: comedi: amplc_dio200.h: remove boardinfo 'mainshift'

This member of the boardinfo is only set for the PCIE boards. Use the
'is_pcie' flag to determine if the offset needs to be shifted when
reading/writing the registers.

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>