spi: davinci: Drop the preload of TX buffer before read/writes operations

spi: davinci: Drop the preload of TX buffer before read/writes operations

A write to the TX buffer is performed before entering the loop to "avoid
clock starvation". This sometimes results in subsequent writes in
davinci_spi_xfer_data() to occur while the TXFULL bit is asserted,
leading to write failures.

Remove the preload of the TX buffer.

Signed-off-by: Bastien Curutchet <bastien.curutchet@bootlin.com>