s390/dasd: Always store parameter elements in an array

When the DASD driver is built into the kernel, the entire comma
separated parameter list is stored as one single element in the dasd[]
array, opposed to the module build where each element is stored
separately in dasd[].
There is no point in doing so. Therefore, store each part of the list as
single elements in dasd[] as well when built into the kernel.
Also, create a define for the maximum of 256 parameters.

Reviewed-by: Stefan Haberland <sth@linux.vnet.ibm.com>
Reviewed-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Jan Höppner <hoeppner@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>