staging: comedi: das1800: prefer kmalloc_array over kmalloc with multiply

Checkpatch doesn't like kmalloc with multiply very much:
drivers/staging/comedi/drivers/das1800.c:1377: WARNING: Prefer kmalloc_array over kmalloc with multiply

So this patch swaps that use out for kmalloc_array instead.

Signed-off-by: Chase Southwood <chase.southwood@gmail.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>