media: dvb-usb: fix uninit-value in vp702x_read_mac_addr

If vp702x_usb_in_op fails, the mac address is not initialized.
And vp702x_read_mac_addr does not handle this failure, which leads to
the uninit-value in dvb_usb_adapter_dvb_init.

Fix this by handling the failure of vp702x_usb_in_op.

Fixes: 786baecf ("[media] dvb-usb: move it to drivers/media/usb/dvb-usb")
Signed-off-by: Dongliang Mu <mudongliangabcd@gmail.com>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>