net: usb: sr9800: fix uninitialized local variable

commit 77b6d09f4ae66d42cd63b121af67780ae3d1a5e9 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>