net: wwan: t7xx: check skb_clone in control TX

In the Linux kernel, the following vulnerability has been resolved: net: wwan: t7xx: check skb_clone in control TX t7xx_port_ctrl_tx() clones each skb fragment before passing it to the port transmit path. The clone is used immediately to set cloned->len, so an skb_clone() failure results in a NULL pointer dereference. Check the clone before using it. If previous fragments were already queued, preserve the driver's existing partial-write behavior by returning the number of bytes submitted so far.