Replace the code that guarantees the device stays in direct mode
with iio_device_claim_direct_mode() which does same.  This allows
removal of an unused lock in the device private global data.

Also prevents a possible race around a buffer being enabled whilst
a read is going on.
Signed-off-by: Alison Schofield <amsfield22@gmail.com>
Cc: Daniel Baluta <daniel.baluta@gmail.com>
Acked-by: Peter Meerwald-Stadler <pmeerw@pmeerw.net>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>