mfd: twl6040: Check for error when reading revision register

We may have an error when reading the revision register, so check for
the returned value.

Acked-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Florian Vaussard <florian.vaussard@epfl.ch>
Signed-off-by: Lee Jones <lee.jones@linaro.org>