xilinx_spips: Don't set TX FIFO UNDERFLOW at cmd done

Don't set TX FIFO UNDERFLOW interrupt after transmitting the commands.
Also update interrupts after reading out the interrupt status.

Signed-off-by: Francisco Iglesias <frasse.iglesias@gmail.com>
Acked-by: Alistair Francis <alistair.francis@xilinx.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Tested-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Message-id: 20171126231634.9531-12-frasse.iglesias@gmail.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
