Allocate correct size (portably) in drivers/usb/gadget/f_midi.c::f_midi_bind()

Allocate correct size (portably) in drivers/usb/gadget/f_midi.c::f_midi_bind()

As the coverity checker puts it:

"Passing argument "sizeof (midi_function) /*8*/" to function "kcalloc"
and then casting the return value to "struct usb_descriptor_header **"
is suspicious. ... In this particular case sizeof(struct
usb_descriptor_header **) happens to be equal to sizeof(struct
usb_descriptor_header *), but this is not a portable assumption."

I believe we really do intend to use 'sizeof(*midi_function)' here, so
this patch makes that change.

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>