usb: dwc3: Replace sscanf with kstrtou8_from_user

Variable "ubuf " and "buf" 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: Ida147ab86b0185260d31b6025179a78de78d1553
Signed-off-by: Liangliang Lu <luliang@codeaurora.org>