usb: dwc3: Replace sscanf with kstrtou8_from_user

Variable "ubuf " may point to  kernel address, a malicious user
could use code logic to get kernel information. Use
kstrtou8_from_user() which take care of copying buffer, and finding
u8 value here.

Change-Id: I7228d893ef757c814b26745663270b8a461b1728
Signed-off-by: Liangliang Lu <luliang@codeaurora.org>