iio: light: bh1780: assign a static name

Using the struct i2c_device->id field for naming the light sensor
is a bad idea: when booting from the pure device tree this is NULL
and that causes the device not to have the "name" property in
sysfs and that in turn confuses the "lsiio" command to stop listing
devices.

So instead of using the device .id, use the hard string "bh1780",
which works just fine.

Fixes: 1f0477f1 ("iio: light: new driver for the ROHM BH1780")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>