spi: s3c64xx: restore removed comments

Patch a9e93e8 has erroneously removed some comments which are
important to understand why the bus frequency is multiplied by
two during the spi transfer.

Reword the previous comment to a more appropriate message.

Suggested-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Andi Shyti <andi.shyti@samsung.com>
Reviewed-by: Michael Turquette <mturquette@baylibre.com>
Signed-off-by: Mark Brown <broonie@kernel.org>