fix sql driver interface for subnet type

Without this fix the interface was not recognized and resulted in an
error message.

types/ip.go

@@ -58,7 +58,7 @@ func (i IP) Is6() bool {
 //
 // This function is needed so that a subnet can be inserted into
 // the database without much casting.
-func (s *Subnet) Value() (driver.Value, error) {
+func (s Subnet) Value() (driver.Value, error) {
 	return s.String(), nil
 }