staging: comedi: ni_mio_common: fix local var for 32-bit read

The local variable 'd' is used to read a 32-bit register. One branch of
the code shifts the register value 16 bits to the right to get the high
16-bits. Change the type of the local variable so that this shift works
correctly.

Reported by: coverity
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>