usb: gadget: f_cdev: Fix memory leak with func_name

func_name related memory is allocated on creating instance of driver with
cser_set_inst_name() API with usage of kstrndup(). If cser_set_inst_name()
returns non-zero value or driver instance is released, this func_name
related memory is not freed. Fix this memory leak by freeing func_name from
cser_free_inst() API.

CRs-Fixed: 1070060
Change-Id: I7ba4c5a65bb01d056b4d2690fe30f7cbe47ba311
Signed-off-by: Mayank Rana <mrana@codeaurora.org>