wimax/i2400m: fix the race condition for accessing TX queue

The race condition happens when the TX queue is accessed by
the TX work while the same TX queue is being destroyed because
a bus reset is triggered either by debugfs entry or simply
by failing waking up the device from WiMAX IDLE mode.

This fix is to prevent the TX queue from being accessed by
multiple threads

Signed-off-by: Cindy H Kao <cindy.h.kao@intel.com>