staging: comedi: cb_das16_cs: cleanup das16cs_ai_rinsn()

Cleanup to analog input read function.

1) Initialize the chan, range, and aref locale variables when
   they are declared.
2) Remove need for the static local variable.
3) Remove the unnecessary cast of inw()'s return value.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Cc: Frank Mori Hess <fmhess@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>