USB: dwc3: debugfs: Add boundary check in dwc3_store_ep_num()

User can pass arguments as part of write to requests and endpoint number
will be calculated based on the arguments. There is a chance that driver
can access ep structue that is not allocated due to invalid arguments
passed by user. Hence fix the issue by having check and return error in
case of invalid arguments.

Change-Id: I060ea878b55ce0f9983b91c50e58718c8a2c2fa1
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>