coresight: tmc-etr: Fix deadlock issue while reading from ETR

If ETR is being read repeatedly, at the same time, trying to reset
source link again and again, sometimes deadlock problems occur.
Beacause reset_source_sink_store holds coresight_mutex and waits
for mem_lock, meanwhile, tmc_read_prepare_etr holds mem_lock and
waits for coresight_mutex.

Change-Id: I8c70575b8a70d820748492313d6232923478ede3
Signed-off-by: Tao,Zhang <taozha@codeaurora.org>