firmware: arm_scpi: decrease Tx timeout to 20ms

Currently we have Tx timeout of 50ms while Rx timeout of 20 ms. Tx state
machine is maintained by the mailbox framework and Rx by SCPI driver.

It is possible that before msg_submit call tx_prepare(because of other
message in the queue and the channel being active), wait for completion
in scpi_send_message times out and the buffers are freed. In that case
when Tx state machine timer goes off later, poll_txdone calls
scpi_tx_prepare on that message, which adds it to the rx_pending list,
corrupting link pointers.

This patch reduces the Tx timeout to 20ms and increases Rx timeout to
30ms to avoid the above mentioned issue.

Reported-by: Jon Medhurst (Tixy) <tixy@linaro.org>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>