drm/nouveau/i2c: bump the i2c delay for the adt7473

Some adt7473 can't manage the 20µs delay we use for the bitbanging, bumping
it to 40µs seem to do the trick.

Signed-off-by: Martin Peres <martin.peres@free.fr>
Tested-by: Marcel Dopita <mdop@seznam.cz>