wifi: libertas: don't kill URBs in interrupt context

In the Linux kernel, the following vulnerability has been resolved:

wifi: libertas: don't kill URBs in interrupt context

Serialization for the TX path was enforced by calling

usb_kill_urb()/usb_kill_anchored_urbs(), to prevent transmission before

a previous URB was completed. usb_tx_block() can be called from

interrupt context (e.g. in the HCD giveback path), so we can't always

use it to kill in-flight URBs.

Prevent sleeping during interrupt context by checking the tx_submitted

anchor for existing URBs. We now return -EBUSY, to indicate there's

a pending request.