coresight-tpda: skip writing '0' to FREQREQ_VAL register

FREQREQ_VAL is configured by RPM at the time of frequency switch.
TPDA enable sequence resets this register if user doesn't provide a
freq_req_val. This can be problem as it can accidentally reset the value
configured by RPM.

Change-Id: I2eb733bac700de372ae680cf74a251bb463e6650
Signed-off-by: Shashank Mittal <mittals@codeaurora.org>