dasd: unexport dasd_set_target_state

dasd_set_target_state is only used inside of dasd_mod.ko, so don't
export it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Stefan Haberland <sth@linux.ibm.com>
Link: https://lore.kernel.org/r/20210701142221.3408680-2-sth@linux.ibm.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>