staging: comedi: serial2000: checkpatch.pl cleanup (kmalloc_array)

Fix the checkpatch.pl warning:

WARNING: Prefer kmalloc_array over kmalloc with multiply

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>