iio: inv_mpu6050: Make interrupt optional

The inv_mpu6050 driver requires an interrupt for buffered capture. But non
buffered reading for measurements works just fine without an interrupt
connected.

Make the interrupt optional to support this case.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Jean-Baptiste Maneyrol <jmaneyrol@invensense.com>
Link: https://lore.kernel.org/r/20210325131046.13383-2-lars@metafoo.de


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>