staging: comedi: dt2801: Prefer using the BIT macro

As suggested by checkpatch.pl, this patch replaces bit shifting on 1 with the
BIT(x) macro.

Signed-off-by: Bhaktipriya Shridhar <bhaktipriya96@gmail.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>