regulator: rk8xx: clarify operator precedence

My linter complains that the order isn't clear enough so let's put
parentheses around the ternary condition to make it happy.

Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>  # chromebook-bob