iio: st_sensors: fetch and enable regulators unconditionally

These sensors all have Vdd and Vdd_IO lines. This means the
supplies are *not* optional (optional means that the supply is
optional in the electrical sense, not the software sense)
so we need to get the and enable them at all times.

If the device tree or board file does not define suitable
regulators for the component, it will be substituted by a
dummy regulator, or, if regulators are disabled altogether,
by stubs. There is no need to use the IS_ERR_OR_NULL() check
that is considered harmful.

Cc: Giuseppe Barba <giuseppe.barba@st.com>
Cc: Denis Ciocca <denis.ciocca@st.com>
Cc: Crestez Dan Leonard <leonard.crestez@intel.com>
Cc: Gregor Boirie <gregor.boirie@parrot.com>
Cc: Mark Brown <broonie@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>