iio:dac:ad5064 mlock cleanup - move to a local lock.

indio_dev->mlock is intended to protect state transitions in
the core. It's scope is tightly defined. For device specific
uses such as those made here, we should define a local lock
allowing the scope of the lock to be defined near to what it
is protecting.

These mlock changes can be non obvious, but given we don't do
anything other than direct for DACs, these ones are easy to do.

If anyone wants to help with this particular effort it would
be most welcome!

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>