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>