drivers/sensor: lsm6dsl: Fix build when irq_gpios is not in DT

In case of h/w setup with multiples device instances it is possible
that some of them wants to use triggers but not the others (no
interrupt line.

Since Kconfig configuration is the same way for all the instances
(CONFIG_LSM6DSL_TRIGGER=y), the driver behaves differently according
to how the device instance has been configured in the DT.
If irq-gpios is present, then the driver initialize the interrupt
part, else it skip irq init and returns ok, but data->gpio != NULL
xis checked in trigger_set() API.

Signed-off-by: Armando Visconti <armando.visconti@st.com>