Initialize usb_count in the setup funtion.

Spurious wakeups can cause the test thread to return even if the
callback was not actually called as the usb_count isnt initialized.

Bug: 65469351
Test: mma and pushed locally to the device & tested.
--skip-preconditions --module VtsHalUsbV1_0Target
Change-Id: Ib0e838cf4a44807142eab6aa5e9df0cc462bb973