According to the AMD BKDG, the GPIO ActiveLevel bits (10:9) map to:
 00 Active High
 01 Active Low
 10 Active on both edges iff LevelTrig (bit 8) == 0
 11 Reserved

The current code has a bug where it interprets 00 => Active Low, and
01 => Active High.

Fix the bug, restrict "Active on both" to just the edge trigger case, and
refactor a bit to make the logic more readable.
Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>