qeth: fix rx checksum offload handling

ethtool is used to change some device driver features
such as RX/TX hardware checksum offloading.
The qeth device driver callback function to
turn on/off RX hardware check sum handling never changes
the hardware configuration.
The NETIF_F_RXCSUM bit is cleared when the feature bitset
type netdev_features_t(64bit) is assigned to 32 a bit
variable.

This patch fixes the NETIF_F_RXCSUM handling.
Also there is no need to manipulate the device's features
bit set as this is done by the caller when no error occurs.

Signed-off-by: Thomas Richter <tmricht@linux.vnet.ibm.com>
Signed-off-by: Ursula Braun <ursula.braun@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>