slimbus: messaging: add slim_alloc/free_txn_tid()

This patch adds slim_alloc_txn_tid() and slim_free_txn_tid() api
to allow controllers like ngd to allocate tids for user specific
commands. This also cleans up the existing code to use single place
for tid allocations and free.

This patch also make the tid allocation cyclic one, its very useful
to track the transactions back during debug.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>