coresight: take lock before modifying byte cntr value

Byte counter overflow is computed based on byte counter value during
byte counter start routine. Two different threads running in parallel
can cause synchronization issue where one thread modifies overflow
based on byte counter value and other thread modifies byte counter
value at the same time. Take respective lock before setting byte
counter value.

Change-Id: I923a34bf918abe4d5e3b0d30ed4887a38db6f427
Signed-off-by: Sarang Joshi <spjoshi@codeaurora.org>