i2c: xiic: Only ever transfer single message

Transferring multiple messages via XIIC suffers from strange interaction
between the interrupt status/enable register flags. These flags are being
reused in the hardware to indicate different things for read and write
transfer, and doing multiple transactions becomes horribly complex. Just
send a single transaction and reload the controller with another message
once the transaction is done in the interrupt handler thread.

Signed-off-by: Marek Vasut <marex@denx.de>
Acked-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>