Staging: comedi: serial2002: Reduce stack usage on 'open'

Reduce stack usage in serial_2002_open() by allocating dig_in_config,
dig_out_config, chan_in_config, and chan_out_config temporary arrays
using kcalloc() and freeing them when done with.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>