iio: st_sensors: Drop the protection on _avail functions

The use of a lock there seems pointless. Besides preventing to read
these information from userspace while buffers are enabled (which is not
supposed to happen), it only protect read accesses over static const
values, which are never supposed to be written anyway.

Drop these lock calls.

Suggested-by: Jonathan Cameron <jic23@kernel.org>
Cc: Denis Ciocca <denis.ciocca@st.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/r/20220207143840.707510-6-miquel.raynal@bootlin.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>