refactor(ti): refactor ti_sci_{setup,do}_xfer to allow zero size response

This allows us to use the common xfer setup path even for no-wait
messages. Then factor that out of each no-wait function.

Signed-off-by: Andrew Davis <afd@ti.com>
Change-Id: Ib17d3facd293f3fc91dda56b2906121b43250261