staging: comedi: style - use kmalloc_array(n, s, ...) instead of kmalloc(n*s, ...)

Fix a style issue reported by checkpatch.pl for the Eudyptula challenge

Signed-off-by: Wolfgang Ocker <weo@weo1.de>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>