staging: comedi: jr3_pci: use kasprintf

kasprintf combines kmalloc and sprintf, and takes care of the size
calculation itself.

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>