coresight-tmc: fix read_count for tmc_open

'read_count' is used to reference count simultaneous TMC read requests.

This count should reset to zero in case tmc_read_preapare fails.

Change-Id: Iaded781ee76ec3b079ac9cd9d551bdfb42834fd9
Signed-off-by: Shashank Mittal <mittals@codeaurora.org>