huawei: Check for AT^RFSWITCH support to determine offline command

huawei: Check for AT^RFSWITCH support to determine offline command

Using AT+CFUN=7 (if supported) to switch into offline mode has a problem
with older modems. Sending AT+CFUN=7 with a data connection active has
the problem that it never terminates with OK or ERROR.

Devices that support AT^RFSWITCH are known to handle this properly. So
only when that command is supported, use AT+CFUN=7 for offline mode. If
it is not supported, then keep using AT+CFUN=5 to switch the device into
offline mode.