staging: comedi: das16: tidy up user ai/ao range initialization

The allocation of the user range tables could fail. Make sure to check
for it.

Change the kmalloc()'s to kzalloc()'s to make sure the allocated range
tables are initialized to a known state.

Change the local variables so they can be used for both the ai and ao
range initialization and use shorter names to keep the lines < 80 chars.

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>