Fix bug where channels 17 and 18 wouldn't trigger properly

[Kagamiin]

Other changes:

• Removed unused variables in ESFM_write_reg_buffered_fast

Tests performed:

• Tested in Furnace Tracker (early indev driver), channels 17 and 18 are now triggering properly.
  • That also fixed a strange readback issue with channel 18's note-on registers where it was reading back garbage data due to apparently indexing the channels array out of bounds.