Makefile: on clean remove also wr_nic objects

Signed-off-by: Federico Vaga <federico.vaga@cern.ch>
Acked-by: Alessandro Rubini <rubini@gnudd.com>

kernel/Makefile

@@ -51,3 +51,4 @@ install modules_install:
 clean:
 	rm -rf *.o *~  .*.cmd *.ko *.mod.c .tmp_versions Module.symvers \
 		Module.markers modules.order
+	rm -rf wr_nic/*.o wr_nic/*~