iio: accel: adxl313_core: Use devm-managed mutex initialization

iio: accel: adxl313_core: Use devm-managed mutex initialization

Use devm_mutex_init() to tie the mutex lifetime to the device and
improve debugging when CONFIG_DEBUG_MUTEXES is enabled.

Signed-off-by: Sanjay Chitroda <sanjayembeddedse@gmail.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>