usb: serial: csvt: Modify interface sub class value for PID 0x904c

mdm9x15 sends interface subclass value as 0xfe during usb device
enumeration for product id 0x904c. Hence change value from 0x0 to 0xfe.

Change-Id: I39f171efd8fe706790529f4ef62887805cf91abe
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>