slimbus: Modify check for empty slots

There was a corner case where the open slot count was going -ve
indicating that there are no slots left for messaging. The current
check to ensure non-zero messaging slots does not take that into
account. This commit fixes that check.

CRs-fixed: 370229
Change-Id: I832ce0a26f1919d3d03b4ef0311603a5a778685e
Signed-off-by: Sagar Dharia <sdharia@codeaurora.org>