iio: proximity: as3935: recalibrate RCO after resume

According to the datasheet the RCO must be recalibrated
on every power-on-reset. Also remove mutex locking in the
calibration function since callers other than the probe
function (which doesn't need it) will have a lock.

Fixes: 24ddb0e4 ("iio: Add AS3935 lightning sensor support")
Cc: George McCollister <george.mccollister@gmail.com>
Signed-off-by: Matt Ranostay <matt.ranostay@konsulko.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>