iio: st_sensors: Use iio_device_claim/release_direct_mode() when relevant

The st_sensors_core driver hardcodes the content of the
iio_device_claim_direct_mode() and iio_device_release_direct_mode()
helpers. Let's get rid of this handcrafted implementation and use the
proper core helpers instead. Additionally, this lowers the tab level
(which is always good) and prevents the use of the ->currentmode
variable which is not supposed to be used like this anyway.

Cc: Denis Ciocca <denis.ciocca@st.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/r/20220207143840.707510-9-miquel.raynal@bootlin.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>