s390/dasd: Make raw I/O usable without prefix support

The Prefix CCW is not mandatory and raw I/O can also be issued without
it. Check whether the Prefix CCW is supported and if not use the
combination of Define Extent and Locate Record Extended instead.

While at it, sort the variable declarations, replace the gotos with
early exits, and remove an error check at the end which is irrelevant.
Also, remove the XRC check as it is not relevant for raw I/O.

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