smsc75xx: fix smsc75xx_set_features()

If an attempt is made to disable RX checksums, USB adapter is changed
but netdev->features is not, because smsc75xx_set_features() returns a
non zero value.

This throws errors from netdev_rx_csum_fault() :
<devname>: hw csum failure

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Steve Glendinning <steve.glendinning@shawell.net>
Signed-off-by: David S. Miller <davem@davemloft.net>