overlays: Correct gpio-fan gpio flags for 4.19

The gpio-fan overlay was submitted for the 4.14 kernel where the second
value in the Device Tree gpios declaration was ignored (thanks to an
old-style driver), allowing the fan-control output to be active-high
even though the declaration appears to request it be active-low.
The gpio-fan driver in 4.19 uses GPIO descriptors and honours the
active-low flag that the overlay (accidentally?) supplies.

Change/correct the flags field to mark the GPIO as active-high.

Signed-off-by: Phil Elwell <phil@raspberrypi.org>