spi: spi-mpc512x-psc: let transmiter/receiver enabled when in xfer loop

There is no need to disable transmitter/receiver after each loop
iteration and re-enable it for next loop iteration. Enable the
transmitter/receiver before xfer loop starts and disable it when
the whole transfer is done.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>