MLK-20004 spi: lpspi: Improve the stability of lpspi data transmission

MLK-20004 spi: lpspi: Improve the stability of lpspi data transmission

Use SR_TDF to judge if need send data, and SR_FCF to judge if
transmission end to replace the waiting after transmission end. This
waiting has no actual meaning, for the real end will set the FCF
flag.

Resolved an issue that could cause a transmission timeout when
transferring large amounts of data.

After making these changes, there is no need to use
fsl_lpspi_txfifo_empty(), so remove it.

Signed-off-by: Xiaoning Wang <xiaoning.wang@nxp.com>
Reviewed-by: Fugang Duan <fugang.duan@nxp.com>
Signed-off-by: Arulpandiyan Vadivel <arulpandiyan_vadivel@mentor.com>