iio: health: max30100: do not use internal iio_dev lock

iio: health: max30100: do not use internal iio_dev lock

The pattern used in this device does not quite fit in the
iio_device_claim_direct_mode() typical usage. In this case,
iio_buffer_enabled() was being used not to prevent the raw
access but to allow it. Hence, let's make use of the new
iio_device_claim_buffer_mode() API to make sure we stay in
buffered mode during the complete read.

Note that we are shadowing the error code returned by
iio_device_claim_buffer_mode() so that we keep the original one
(-EAGAIN). The reason is that some userspace stack might already be
relying on this particular code so that we are not taking chances and
leave it alone.

Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20221012151620.1725215-3-nuno.sa@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>