[MusicXML export] incorrect display-step and display-octave for notes on percussion staff with lines unequal to five

When exporting notes on percussion staff (using percussion clef), if the number of staff lines is not equal to five, display-step and display-octave are incorrect.

Example: for a one-line percussion staff, display-step must be E, display-octave must be 4. Currently MuseScore generates F 5.