media: pvrusb2: fix array-index-out-of-bounds in pvr2_i2c_core_init

Syzbot reported that -1 is used as array index. The problem was in
missing validation check.

hdw->unit_number is initialized with -1 and then if init table walk fails
this value remains unchanged. Since code blindly uses this member for
array indexing adding sanity check is the easiest fix for that.

hdw->workpoll initialization moved upper to prevent warning in
__flush_work.

Reported-and-tested-by:  <syzbot+1a247e36149ffd709a9b@syzkaller.appspotmail.com>

Fixes: d855497e ("V4L/DVB (4228a): pvrusb2 to kernel 2.6.18")
Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>