power: qpnp-charger: only monitor bat_therm when bat_if present

If the battery interface peripheral is disabled, JEITA
notifications are not required.

Fix this by checking if the bat_if base address is
initialized before requesting vadc notifications.

Change-Id: I8b2b3132771879fce07c64d55b6fe614e20ce790
Signed-off-by: David Keitel <dkeitel@codeaurora.org>