coresight: fix use-after-free in stm on secure boot devices

When the STM driver failed to register, the stmdrvdata variable becomes
dangling pointer which may cause APSS to crash.

Change-Id: I0e709ac180d8c946f25e026b23edadb08c82a1f3
Signed-off-by: Charan Teja Reddy <charante@codeaurora.org>