firmware: silence __fw_modbuild and __fw_modinst 'Nothing to be done' messages

People don't like them and think they're errors.

Leave the __fw_install one though; when 'make firmware_install' does
nothing, it's best to have a 'Nothing to be done for...' message rather
than just doing nothing.
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>

scripts/Makefile.fwinst

@@ -50,8 +50,12 @@ PHONY +=  __fw_install __fw_modinst FORCE
 .PHONY: $(PHONY)
 
 __fw_install: $(installed-fw)
+
 __fw_modinst: $(installed-mod-fw)
+	@:
+
 __fw_modbuild: $(addprefix $(obj)/,$(mod-fw))
+	@:
 
 FORCE: