If cdev_add() returns an error, the code calls
cdev_del() passing the STm->cdevs[rew] pointer as parameter;
the problem is that the pointer has not been initialized yet.

This patch fixes the problem by moving the STm->cdevs[rew] pointer
initialization before the call to cdev_add().
It also sets STm->devs[rew] and STm->cdevs[rew] to NULL in
case of failure.
Signed-off-by: NMaurizio Lombardi <mlombard@redhat.com>
Reviewed-by: NJohannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: NTomas Henzl <thenzl@redhat.com>
Signed-off-by: NMartin K. Petersen <martin.petersen@oracle.com>