switch: Reset device's drvdata before destroying it

dev_set_drvdata() sets the device specific driver data at
dev->p->driver_data.  However, dev is destroyed and has it's
context memory freed in device_destroy().  As a result, calling
dev_set_drvdata() causes a fault when accessing dev->p.

Change-Id: Id6cc72e1c635d6c4e69db3404bf57ebbab35d464
Signed-off-by: Deva Ramasubramanian <dramasub@codeaurora.org>