MLK-21153-1 spi: lpspi: fix the dataloss when slave is in PIO mode

Related issues have been fixed by (e0e542ae MLK-20060-1 spi: lpspi:
fix wrong transmission when don't use CONT). However, the delay between
sending and receiving in slave mode. 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>
(cherry picked from commit 1a260d13aa3724b9919a2d8e2f53b4ad26ccf372)