USB: gadget eth: Fix calculate CRC32 in EEM

CRC should be calculated for Ethernet frame, not for whole recievede EEM data.
This bug shows rarely, because in many times len == skb->len.

Signed-off-by: Jiri Pinkava <jiri.pinkava@vscht.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>