iio:st_pressure: clean useless static channel initializers

Some static channels are explicitly initialized with default values.
Remove them to enhance readability.

Signed-off-by: Gregor Boirie <gregor.boirie@parrot.com>
Tested-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>