iio: gyro: adis16130: remove mlock usage

The use of indio_dev's mlock is discouraged. The driver already defines
it's own `bus_lock` in `adis16130_spi_read()`, so using the mlock is
redundant.

The parts supported by this chip are obsoleted anyway, so for now we just
remove mlock as part of a general cleanup, until the driver gets removed.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>