staging: comedi: ni_at_ao: using the BIT(x) macro

Fix the checkpatch.pl issue:
CHECK: Prefer using the BIT macro
replacing bit shifting on 1 with the BIT(x) macro.

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