st: fix potential null pointer dereference.

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: Maurizio Lombardi <[email protected]>
Reviewed-by: Johannes Thumshirn <[email protected]>
Reviewed-by: Tomas Henzl <[email protected]>
Signed-off-by: Martin K. Petersen <[email protected]>