spi: sunxi: fix clock divider calculation for max frequency setting

If the maximum frequency is requested, we still fall into the CDR2
handling. But there the minimal divider is 2. For the sun6i and sun8i we
can do better with the CDR1 setting where the minimal divider is 1:
  SPI_CLK = MOD_CLK / 2 ^ cdr with cdr = 0

Thus, handle the div = 1 case specially.

While at it, correct the comment above the calculation.

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