media: gspca/sq905.c: fix uninitialized variable

act_len can be uninitialized if usb_bulk_msg() returns an error.
Set it to 0 to avoid a KMSAN error.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reported-by:  <syzbot+a4e309017a5f3a24c7b3@syzkaller.appspotmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>