staging: comedi: use kmalloc_array instead of kmalloc

Use kmalloc_array instead of kmalloc to allocate memory for an array.

Signed-off-by: Geliang Tang <geliangtang@163.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>