cdc-eem: bad crc checking

When the driver received an EEM packet with CRC option enabled, driver must
compute and check the CRC of the Ethernet data. Previous version computes CRC
on Ethernet data plus the original CRC value. Skbuff is correctly trimed but
the old length is used when CRC is computed.

Signed-off-by: Vincent CUISSARD <vincent.cuissard@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>