usb: gadget: udc-core: Do not report -EISNAM error from gadgetfs

When mounting a gadgetfs the following error message is seen:

$ modprobe gadgetfs
gadgetfs: USB Gadget filesystem, version 24 Aug 2004
$ mkdir  /dev/gadget
$ mount -t gadgetfs none /dev/gadget
nop ci_hdrc.0: failed to start (null): -120

The error comes from gadgetfs_probe(), which returns -EISNAM (-120).

As Alan Stern explains[1], this is the normal behavior:

"It is a temporary measure, used only when the file system is set up
initially.  The real bind routine is gadgetfs_bind(), which gets called
when userspace configures the gadget.

In short, this is how it is intended to work.  It isn't a bug."

[1] http://marc.info/?l=linux-usb&m=138029668707075&w=2



So in order to prevent the error message, do not report EISNAM as an error.

Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>