iface-modem: don't fail enabling if modem_power_up() isn't implemented

There are modems (e.g. Nokia, Thuraya, Iridium) which don't require or don't support power management, and therefore there is no way to either load or update the power status.

In those modems we just assume ON is the current and only value (set in the skeleton during initialization) and so when we attempt to update the power state to ON during enabling, the logic should not break.

Fix the logic by making sure the requested_power_setup() function pointers are only checked for validity after ensuring we're not already in the desired power state.

Fixes #398 (closed)