iio:accel:stk8312: Add lowercase i2c device id

These are never upper case.  Chances are that all users of this driver
were using the ACPI binding but just in case keep the uppercase version
but mark it deprecated.

Whilst here tidy up some spacing.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/20210401144226.225928-1-jic23@kernel.org