coresight: stm: Fix null point issue in stm

use-after-free over the global variable .stmdrvdata cause NULL point crash.
stm_register_device call fail will be deferred probe, this action will
release all devm_** managed resource, so the preset stm_set_ost_params
will be cleared, in this case, if any call function use the global
.stmdrvdata variable will cause problem.

Change-Id: I8b287389f47c0b131f26ff00eef3f4c1bb7e63cd
Signed-off-by: Mulu He <muluhe@codeaurora.org>