soc: qcom: Change the way to locate log_buf_len

Use the exported function instead of kallsyms_lookup to get
log_buf size, this can save a lot of time during boot up.

Change-Id: I99fe49e9f014c0a4bb16162d836c38e31236e8ea
Signed-off-by: Mayank Grover <groverm@codeaurora.org>