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>