core: Suppress hotplug events during initial enumeration

core: Suppress hotplug events during initial enumeration

The initial enumeration should not result in hotplug events to be fired.
This is just a convenience though, API users still need to be prepared
to be notified a second time for a device that was plugged in between
libusb_init and libusb_hotplug_register_callback.

This regressed with commit 6929b82 ("Fix segmentation fault in
libusb_init() if usbi_backend.init() fails"). This commit avoids the
mentioned segmentation fault by avoiding to clean up the hotplug code if
it was not yet initialised.

Fixes #1082
Closes #1090
References #989

[Cherrypick from mainline commit 0846456 ("core: Suppress hotplug events during initial enumeration")]
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Change-Id: Idbae2c394ba01694f9e6006d8a52036f6c73da77