[NET]: Add __dev_getfirstbyhwtype

Add __dev_getfirstbyhwtype for callers that don't want a reference but
some data from the device and thus need to take the rtnl anyway.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>