staging: et131x: on transmit, stop the queue if the next packet will fail

* Currently the tx queue is only stopped when the current packet fails.
Check if the next packet will fail, and stop the queue if so.

* Removed associated item from TODO list in the README.

* Also minor fixup as adapter was being declared as null and immediately set
to a value.

Signed-off-by: Mark Einon <mark.einon@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>