staging: vt6656: don't return zero on failure path in vt6656_probe()

If ieee80211_alloc_hw() fails in vt6656_probe(), it breaks off
initialization, but returns zero.

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>