drivers: spi: mcux_flexio: Fix flexio SPI loop delay issue

The function FLEXIO_SPI_MasterTransferNonBlocking has a bug when
works in CS continuous mode. In this mode, both RX and TX
interrupts are enabled, they share the same IRQ line.

In the ISR, the RX event and TX event are not handled well,
so a short delay in ISR is needed.

The function FLEXIO_SPI_MasterTransferNonBlocking issue is fixed,
the delay in ISR can be removed.

Signed-off-by: Jason Yu <zejiang.yu@nxp.com>