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.

CRs-Fixed: 449732
Change-Id: I4b1e6d17db52d0e31caf9b52e59979ff9cbb0c12
Signed-off-by: Deva Ramasubramanian <dramasub@codeaurora.org>
Signed-off-by: Shivaraj Shetty <shivaraj@codeaurora.org>