spi: mvebu: only read from bus when data is needed
This optimization reduced the time to transfer 40 MB at 50 MHz from
214 seconds to 203 seconds.
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>