net: usb: sr9800: fix uninitialized local variable

Make sure res does not contain random value if the call to
sr_read_cmd fails for some reason.

Reported-by:  <syzbot+f1842130bbcfb335bac1@syzkaller.appspotmail.com>
Signed-off-by: Valentin Vidic <vvidic@valentin-vidic.from.hr>
Signed-off-by: David S. Miller <davem@davemloft.net>