imon_probe() does three usb_get_dev(), but there is no any
usb_put_dev() in the driver.

The patch adds usb_put_dev() to error paths, to imon_disconnect()
and to imon_probe() as far as usbdev is not saved anywhere.

Found by Linux Driver Verification project (linuxtesting.org).
Signed-off-by: NAlexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: NMauro Carvalho Chehab <mchehab@osg.samsung.com>