iio: imu: inv_mpu6050: fix lock issues by using our own mutex

There are several locks issues when using buffer and direct polling
data at the same time. Use our own mutex for managing locking and
block simultaneous use of buffer and direct polling by using
iio_device_{claim/release}_direct_mode. This makes chip_config
enable bit obsolete, so delete it.

Signed-off-by: Jean-Baptiste Maneyrol <jmaneyrol@invensense.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>