The lock is used to protect the buffer during reads. Though the spi
routines have their own locks, it may be the case that the buffer needs to
be protected before it's stored and passed to the IIO read hooks.

indio_dev's mlock was used before.
This change replaces it with the driver's own lock.
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>