usb: gadget: f_midi: allow customizing the USB MIDI interface string through configfs

usb: gadget: f_midi: allow customizing the USB MIDI interface string through configfs

When using f_midi from configfs the USB MIDI interface string is hardcoded
to 'MIDI function'.

This USB string descriptor is used by some third-party OS or software to
display the name of the MIDI device

Since we add an additional string option a new macro block was created to
factorize declarations

Signed-off-by: Victor Krawiec <victor.krawiec@arturia.com>
Reviewed-by: Takashi Iwai <tiwai@suse.de>
Link: https://patch.msgid.link/20251209164006.143219-1-victor.krawiec@arturia.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>