wimax/i2400m: on device stop, clean up pending wake & TX work

When the i2400m device needs to wake up an idle WiMAX connection, it
schedules a workqueue job to do it.

Currently, only when the network stack called the _stop() method this
work struct was being cancelled. This has to be done every time the
device is stopped.

So add a call in i2400m_dev_stop() to take care of such cleanup, which
is now wrapped in i2400m_net_wake_stop().

Signed-off-by: Inaky Perez-Gonzalez <inaky@linux.intel.com>