media: dvb-usb-v2: lmedm04: Improve logic checking of warm start

Warm start has no check as whether a genuine device has
connected and proceeds to next execution path.

Check device should read 0x47 at offset of 2 on USB descriptor read
and it is the amount requested of 6 bytes.

Fix for
kasan: CONFIG_KASAN_INLINE enabled
kasan: GPF could be caused by NULL-ptr deref or user memory access as

Reported-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Malcolm Priestley <tvboxspy@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>