iio: sensors-core: st: Check st_sensors_set_drdy_int_pin()'s return value

Value from st_sensors_set_drdy_int_pin() is assigned to err here,
but that stored value is not used before it is overwritten.  To fix
this we're enforcing a check on st_sensors_set_drdy_int_pin()'s
return value and if it's an error, we're returning right away.

Cc: jic23@kernel.org
Cc: linux-iio@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>