usb: gadget: fix: Call usb_gadget_connect() for dummy_udc

usb: gadget: fix: Call usb_gadget_connect() for dummy_udc

Ugly hack which comes from Android removes a call
to usb_gadget_connect() in udc_bind_to_driver() to
prevent android/slp gadget from communication before
userspace explicitly enables it.

Mainline gadgets (like g_ffs) expect this function
to be called as they don't have any sysfs interface
to enable them later.

As for usb-host API tests we need to use dummy_hcd
and g_ffs let's call usb_gadget_connect() for all
dummy_udc's.

Change-Id: I782bbb51c54e0b87ff6ef976070b6d8870a1a745
Signed-off-by: Krzysztof Opasiak <k.opasiak@samsung.com>