drivers: nxp_flexio_spi: Fix transfer failures

This fixes failures seen with the SPI loopback test.
The fix waits for the TX and RX side to be complete
i.e when RX SHIFTBUF has been loaded from the RX Shifter
and the TX SHIFTBUF has transferred to the TX Shifter.

Signed-off-by: Mahesh Mahadevan <mahesh.mahadevan@nxp.com>