staging: iio: accel: adis16201: remove iio_dev mlock

In the driver adis16201 read raw does not require an iio_dev->mlock for
reads. It can run concurrently as adis_read_reg_16() is protected by a
transaction lock.

Signed-off-by: Aishwarya Pant <aishpant@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>