staging: gdm724x: fix leak at failure path in gdm_usb_probe()

Error handling code in gdm_usb_probe() deallocates all resources,
but calls usb_get_dev(usbdev) and returns error code after that.

The patch fixes it and, by the way, several other issues:
- no need to use GFP_ATOMIC in probe();
- return -ENODEV instead of -1;
- kmalloc+memset -> kzalloc

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>