media: dvb-usb: opera1: fix uninit-value in dvb_usb_adapter_dvb_init

If opera1_xilinx_rw fails, the mac address is not initialized.
And opera1_read_mac_address does not handle this failure, which leads to
the uninit-value in dvb_usb_adapter_dvb_init.

Fix this by handling the failure of opera1_xilinx_rw.

Signed-off-by: Zhang Shurong <zhang_shurong@foxmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>