spi: zynq_spi: Fix infinite looping while xfer

During spi transfer, for example:
sspi 1:1.0 8 ff

the rx_len values will  be:
rx_len = 0
rx_len = 4294967295

This caused a busy looping during xfer, this patch fixes it
by adding a check while reading the rx fifo

Signed-off-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Cc: Michal Simek <michal.simek@xilinx.com>
Cc: Siva Durga Prasad Paladugu <sivadur@xilinx.com>
Reviewed-by: Jagan Teki <jteki@openedev.com>

drivers/spi/zynq_spi.c

@@ -230,7 +230,7 @@ static int zynq_spi_xfer(struct udevice *dev, unsigned int bitlen,
 
 		/* Read the data from RX FIFO */
 		status = readl(&regs->isr);
-		while (status & ZYNQ_SPI_IXR_RXNEMPTY_MASK) {
+		while ((status & ZYNQ_SPI_IXR_RXNEMPTY_MASK) && rx_len) {
 			buf = readl(&regs->rxdr);
 			if (rx_buf)
 				*rx_buf++ = buf;