Staging: rt2870: Add USB ID for Sitecom WL-608

Add the USB id 0x0DF6,0x003F to the rt2870.h file such that the
Sitecom WL-608 device will be recognized by this driver.

Signed-off-by: Jorrit Schippers <jorrit@ncode.nl>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>