uqmi: use the autoconnect feature

Instead of connecting once and saving the packet data handle, let the
firmware handle connecting/reconnecting automatically. This is more
reliable and reduces reliance on potentially stale data.

Use the global packet data handle to attempt to disable autoconnect
before restarting the connection. This ensures that the firmware will
take the new APN/auth settings.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

SVN-Revision: 42721