usb: gadget: f_midi: allow resetting index option

Unlike the other integral options supported by the MIDI gadget, index is
a signed integer and defaults to -1, which means "choose any sound card
index".  The generic store routine parses input into a u32 which fails
to properly read -1 if an attempt is made to set the value to the
default.

Add a new macro block for signed values to fix this, and use the correct
format string for unsigned values.

Signed-off-by: John Keeping <john@metanate.com>
Link: https://lore.kernel.org/r/20211029134115.351008-1-john@metanate.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>