soc: qcom: jtagv8-etm: validate driver data before using

msm_jtag_etm_save_state() and msm_jtag_etm_restore_state() are
called from hotcpu callback notifiers. These notifiers can get
called before jtagv8-etm driver data is initialized in probe()
function. Add NULL pointer check to make sure driver data is
initialized before using it.

Change-Id: Ieb17708a6d07d3928d56da08b275633f003d9ed0
Signed-off-by: Sarangdhar Joshi <spjoshi@codeaurora.org>