[ALSA] usb-audio: simplify MIDI quirk handling

Modules: USB generic driver

Simplify the handling of MIDI quirks by treating an interface without
quirks as a QUIRK_MIDI_STANDARD_INTERFACE.

This also fixes the bug where a MIDI_STANDARD quirk would not be
recognized.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>