media: igorplugusb: use correct size pass to igorplugusb_probe()

After 'buf_in' change to pointer, the sizeof() is not correct buffer
size, it should be MAX_PACKET.

Fixes: b3f820b9 ("media: igorplugusb: respect DMA coherency")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>