coresight: initialize buf during tmc etr probe

Initialize buf for TMC ETR to the virtual address of the allocated
buffer memory. This will ensure a read on TMC ETR device node when ETF
is the current sink doesn't fail.

Change-Id: Id23dd9502ec728567c5b137354ca2d734f9309dc
Signed-off-by: Pratik Patel <pratikp@codeaurora.org>