usb: gadget: f_cdev: Remove func_name and port_num attributes

func_name and port_num attributes related information is already
available as part of created driver instance name itself. These
attributes don't give any valuable information. Hence remove both
attributes and related functionality. Add check against length of
func_name.

CRs-Fixed: 1069476
Change-Id: Ibfc050cda62cfc7e5838949ae833a9d7c0a59303
Signed-off-by: Mayank Rana <mrana@codeaurora.org>