s390/dasd: Improve dasd format code

- Make sure a calling function can rely on data in fdata by resetting to
  its initial values
- Move special treatment for track 0 and 1 to dasd_eckd_build_format
- Replace dangerous backward goto with a loop logic
- Add define for number that specifies the maximum amount of CCWs per
  request and is used for format_step calculation
- Remove unused variable

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