drivers: mailbox: qti_tcs: Return ETIMEDOUT instead of EIO

Respond with a more appropriate error value instead of the generic EIO,
when there is no response from the always-on subsystem.

Change-Id: I11fc2468a4281ed053c571fcbbf331cdcad37a5e
Signed-off-by: Lina Iyer <ilina@codeaurora.org>