s390/dasd: remove dasd_fba_probe() wrapper

commit e03c5941

 ("s390/dasd: Remove unused parameter from
dasd_generic_probe()") allows us to wire the generic callback up
directly, avoiding the additional level of indirection.

While at it also remove the forward declaration for the dasd_fba_driver
struct, it's no longer needed.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Reviewed-by: Jan Hoeppner <hoeppner@linux.ibm.com>
Signed-off-by: Stefan Haberland <sth@linux.ibm.com>
Link: https://lore.kernel.org/r/20210316094513.2601218-2-sth@linux.ibm.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>