[PATCH v4 2/2] Add generic serial MIDI driver using serial bus API

[Daniel Kaehn <kaehndan@xxxxxxxxx>]

Just discovered a fairly severe concurrency bug with the MIDI output
in this version of the driver - write_wakeup and output_write
can end in a deadlock with the serial device. Replying here just to
ensure this isn't merged before I send a fix.

Daniel Kaehn