spi/bcm63xx: fix clock configuration selection

We are currently using an inferior or equal operator for comparing
the transfer frequency with the clock frequency table. Because of
this, we always end up selecting 20Mhz as a frequency, due to the
inequality transfer hz <= 20 Mhz being always true. Fix this by
reversing the inequality, which is how the comparison should be done.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>