slim_ngd: Make sure transaction is not dereferenced after it's freed

Every read transaction over slimbus is a set of one transmit and one
receive. Receive part frees meta-data for the read transaction since
it is no longer needed, and the transaction-ID can be reused.
Make sure that the transmit function doesn't try to dereference the
transaction meta-data after possibly receiving a response.

CRs-fixed: 515802
Change-Id: Id736ad368d4944a6aabc89f4ff295d664bc5efa0
Signed-off-by: Sagar Dharia <sdharia@codeaurora.org>