staging: comedi: addi-data: Don't overwrite read-only data

The drivers for ADDI-DATA cards can override some static parameters for
the board type using information read from EEPROM.  Unfortunately, they
currently write the parameters from the EEPROM back to the shared,
read-only board data!  The problem has been masked during compilation by
type-casting away the const-ness of the data.

This patch changes the code to use an area in the private data for the
board instance to hold the parameters read from EEPROM (after
initializing the parameters from the static board data).  It also
changes the type-casts to the read-only data to preserve the const
qualifier.

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