staging/comedi/adl_pci8164: Don't assign string

Assigning a string is really bad,
and since we only have 1 char strings here we
can simply use a char to store the value and change the format specifier.

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>