staging: comedi: ni_stc.h: simplify MSeries_AI_Config_Bank_Bits()

Simplify this helper function by using a bit-shift to set the high
banks select bit.

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>