How to use Generic USB Joysticks in Android via OTG?
You need to root your device. 
Also, you need to have and install the following:

Software (APK)
1. USB/BT Joy Center

Hardware 
1. USB hub with power capability
2. your generic gamepad
3. USB OTG


After the installation of the APK Insert your hub with power capability (some tablets and phones cant give enough power to the joystick).
When the USB hub lights up it is detected by the tab, all you need to do is plug your generic gamepad.
Now open USB/BT Joy Center this software will automatically detect your gamepad.
(trouble shooting) when your device is not detected. Restart your device with all the hardware connected.
When your tab is already open, try reopening USB/BT Joy Center. I think this should do the trick.