iface-modem: reload current bands after setting

Setting bands is a very device-specific operation. Sometimes the device requires specific band combinations, or sometimes the 'any' specific logic doesn't apply to all supported bands (e.g. may apply only to the currently selected modes, as in XMM based devices).

So, don't assume that if the set command succeeds we have set all expected bands. Instead, do an explicit loading of the current bands after the set operation, same thing as we do when setting modes.