drivers/sensor: lis2mdl: Fix errors when CONFIG_PM_DEVICE=y
drivers/sensor: lis2mdl: Fix errors when CONFIG_PM_DEVICE=y

Fix a couple of issues related to Power Management:

    1. A build error because 'dev' was used even if not declared
       in the caller routine scope
    2. The lis2mdl power management init specific routine was
       not declared in in the device instance definition

Signed-off-by: Armando Visconti <d93ca24ded08d03c3aa60af6d2fd0eb7ce54da11@st.com>

galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr