iio:cros_ec_sensors: Swap from a select to a depends in Kconfig

Would have merged this into the original patch as a fixup but I've already
pushed that out as an immutable branch for others to use so it'll have
to be a separate patch.  The original select had a typo as well.

Trying to do this via a select was opening a can of worms due to
a tree of other elements that would also have needed selecting.
A simple depends seems much mroe straight forward and appropriate in this
case.

Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Cc: Lee Jones <lee.jones@linaro.org>
Cc: Enric Balletbo i Serra <enric.balletbo@collabora.com>