spi: lpspi: fix dataloss when SS is inactivated between every words

If we don't use CONT to keep SS activated or use DMA mode without
cs-gpio, SS will be inactivated between every words. The word here
means the data sent once which length can be set as 1/2/4 bytes.

In the isr function, we read the FSR_RXCOUNT just behind the
fsl_lpspi_read_rx_fifo. This causes the value of FSR_RXCOUNT cannot
reflect whether there is still data not sent timely. So do this
judgement by FSR_TXCOUNT.

Signed-off-by: Clark Wang <xiaoning.wang@nxp.com>
Signed-off-by: Mark Brown <broonie@kernel.org>