mixer_module/tap_esc: fix usage of constrain() on input

[MaEtUgR]

Describe problem solved by this pull request
I changed the input constraint in #15349 but screwed up the usage because I was convinced it's püass by reference. I'll double check for sure next time.

Describe your solution
It's pass by value and the return value has to be assigned again like @bkueng pointed out in https://github.com/PX4/Firmware/pull/15349/files#r457096522 Thanks!!

[bkueng]

@julianoes have you seen this?

 [mavsdk_tests] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
[mavsdk_tests] mavsdk_tests is a Catch v2.9.2 host application.
[mavsdk_tests] Run with -? for options
[mavsdk_tests] -------------------------------------------------------------------------------
[mavsdk_tests] Fly square Multicopter Missions
[mavsdk_tests] -------------------------------------------------------------------------------
[mavsdk_tests] ../../../test/mavsdk_tests/test_multicopter_mission.cpp:55
[mavsdk_tests] ...............................................................................
[mavsdk_tests] ../../../test/mavsdk_tests/test_multicopter_mission.cpp:70: FAILED:
[mavsdk_tests]   {Unknown expression after the reported line}
[mavsdk_tests] due to a fatal error condition:
[mavsdk_tests]   SIGSEGV - Segmentation violation signal
[mavsdk_tests] ===============================================================================

[dagar]

If anyone has this hardware on hand we should do a quick pass updating tap_esc to use OutputModuleInterface.