mfd: stmfx: Improve error message triggered by regulator fault in .remove()

Returning a non-zero value in an i2c remove callback results in the i2c
core emitting a very generic error message ("remove failed (-ESOMETHING),
will be ignored") and as the message indicates not further error handling
is done.

Instead emit a more specific error message and then return zero in
.remove().

The long-term goal is to make the i2c remove prototype return void, making
all implementations return 0 is preparatory work for this change.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20220214150710.312269-1-u.kleine-koenig@pengutronix.de