usb: gadget: android: Reset next_string_id before enable

Reset next_string_id to 0 before enabling the gadget driver. Otherwise,
after a large number of enable/disable cycles, bind will fail
because we cannot allocate new string ids. String ids cannot be larger
than 254 per USB spec.

Change-Id: I44f5fece45008b7a0a18c025d4eb5ce842585c28
Signed-off-by: Benoit Goby <benoit@android.com>