usbnet: ipheth: Remove unnecessary NULL pointer check

ipheth_carrier_set() is called from two locations. In
ipheth_carrier_check_work(), its parameter 'dev' is set with
container_of(work, ...) and can not be NULL. In ipheth_open(),
dev is extracted from netdev_priv(net) and dereferenced before
the call to ipheth_carrier_set(). The NULL pointer check of dev
in ipheth_carrier_set() is therefore unnecessary and can be removed.

Cc: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: David S. Miller <davem@davemloft.net>