iio: gyro: adxrs290: Insert missing mutex initialization call

Insert a missing mutex_init() call during the probe that initializes
the driver's local lock to unlocked state.

Fixes: 2c8920ff

 ("iio: gyro: Add driver support for ADXRS290")
Signed-off-by: Nishant Malpani <nish.malpani25@gmail.com>
Link: https://lore.kernel.org/r/20200825124552.11155-1-nish.malpani25@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>