usb: gadget: f_midi: Use 1024 bytes as MIDI buffer size

Some UDC driver doesn't allow or throw errors when USB request buffer
queued is unaligned i.e. buffer size is not equal to wMaxPacketSize or
multiple of wMaxPacketSize. Fix this issue by using 1024 bytes as MIDI
buffer size instead of 256 bytes.

Change-Id: I71d520a5a800a3e612dc4d57e7baffa93fad61b3
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>