spi: mvebu: fix baudrate calculation for armada variant

The calculation of SPR and SPPR doesn't round correctly at several
places which might result in baud rates that are too big. For example
with tclk_hz = 250000001 and target rate 25000000 it determined a
divider of 10 which is wrong.

Instead of fixing all the corner cases replace the calculation by an
algorithm without a loop which should even be quicker to execute apart
from being correct.

Fixes: df59fa7f

 ("spi: orion: support armada extended baud rates")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Mark Brown <broonie@kernel.org>