iio: st_accel: fix interrupt handling on LIS3LV02

This accelerometer accidentally either emits a DRDY signal or an
IRQ signal. Accidentally I activated the IRQ signal as I thought
it was analogous to the interrupt generator on other ST
accelerometers. This was wrong. After this patch generic_buffer
gives a nice stream of accelerometer readings.

Fixes: 3acddf74

 "iio: st-sensors: add support for lis3lv02d accelerometer"
Cc: Denis CIOCCA <denis.ciocca@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>