usb: gadget: ffs: fix enable multiple instances

This patch fixes an "off-by-one" bug found in
581791f (FunctionFS: enable multiple functions).

During gfs_bind/gfs_unbind the functionfs_bind/functionfs_unbind should be
called for every functionfs instance. With the "i" pre-decremented they
were not called for the zeroth instance.

Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Cc: <stable@vger.kernel.org>
[ balbi@ti.com : added offending commit's subject ]

Signed-off-by: Felipe Balbi <balbi@ti.com>