coresight: tmc: Avoid reading tmc node before it being enabled

It is possible that the mem_size of tmc_etr will be allocated failed
while enabling. And if we read tmc-etr after this, system reset will
happen. So don't read tmc before it being enabled.

CRs-Fixed: 2243995
Change-Id: I6a0be80834f7d4c1c7464f9684228e5b35f31a77
Signed-off-by: Mao Jinlong <jinlmao@codeaurora.org>