spi: sunxi: fix CDR2 calculation

The CDR2 divider calculation always yield a frequency greater than the
requested one. Use DIV_ROUND_UP() to keep the frequency equal or below
the requested one. This way, we can also drop the "if div > 0" check
because we know for a fact that div cannot be zero.

FWIW, this aligns the CDR2 calculation with the linux driver.

Suggested-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Michael Walle <mwalle@kernel.org>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>