nb/intel/sandybridge: Improve CAS freq selection

The previous code seemed weird and tried to check if its selected
value is supported three times.

This also lower the clock if a selected frequency does not result in a
supported CAS number.

Change-Id: I97244bc3940813c5a5fcbd770d71cca76d21fcae
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/19716
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>
2 files changed