usb: gadget: storage: remove nofua file when unbinding

The dev_attr_nofua file was created during fsg_bind() but
was never removed.  Made it a bit more symmetrical and added
code to remove the file in fsg_unbind().

Signed-off-by: Michal Nazarewicz <m.nazarewicz@samsung.com>
Acked-by: Andy Shevchenko <ext-andriy.shevchenko@nokia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>