xilinx_spips: Correct usage of an uninitialized local variable

Coverity found that the variable tx_rx in the function
xilinx_spips_flush_txfifo was being used uninitialized (CID 1383841). This
patch corrects this by always initializing tx_rx to zeros.

Signed-off-by: Francisco Iglesias <frasse.iglesias@gmail.com>
Message-id: 20180124215708.30400-1-frasse.iglesias@gmail.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
