AGESA hudson: Fix SPI writes

Only yangtze has longer FIFO in SPI controller. This was overlooked
in commit

   9f0a2be AMD SPI: Optimise for longer writes

which broke SPI writes and caused CBFS errors with fam15tn.

Change-Id: I821e3f1fa186d2383b30eab9c5d52797c2ef22c5
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/6273
Tested-by: build bot (Jenkins)
Reviewed-by: Idwer Vollering <vidwer@gmail.com>