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

In the Linux kernel, the following vulnerability has been resolved: 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.