slimbus: Initialize data structure before using it

Initialize the data structure in the TX path before using it, so that
it contains appropriate value even under error scenario.

CRs-Fixed: 2002163
Change-Id: Ic58998812843d7d466124f1695a67b1df885fdf5
Signed-off-by: Karthikeyan Ramasubramanian <kramasub@codeaurora.org>