usb: gadget: g_ffs: remove a reduntant gfs_ether_setup variable

Since d6a01439
usb: gadget: move the global the_dev variable to their users
"the_dev" variable can be used as a "setup done" flag; non-NULL
meaning "setup done", NULL meaning "setup not done". Moreover,
gether_cleanup() can be safely called with a NULL argument.

Corrected a comment to be consistent with the code.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>