slimbus: Check the input value for valid use-case

The number of messages to be transferred in a message list is not checked
for the valid use-case scenario.

Check the number of messages for the valid use-case scenario. Also ensure
that the return value is initialized properly.

CRs-Fixed: 2002163
Change-Id: Ic52a2bf83e9423594f3b0307d2428a0fe26cb716
Signed-off-by: Karthikeyan Ramasubramanian <kramasub@codeaurora.org>