Nullify stmfx->vdd in case devm_regulator_get_optional() returns an error.
And simplify code by returning an error only if return code is not -ENODEV,
which means there is no vdd regulator and it is not an issue.

Fixes: d75846ed ("mfd: stmfx: Fix dev_err_probe() call in stmfx_chip_init()")
Signed-off-by: Amelie Delaunay <amelie.delaunay@foss.st.com>
Link: https://lore.kernel.org/r/20230609092804.793100-2-amelie.delaunay@foss.st.comSigned-off-by: Lee Jones <lee@kernel.org>