usb: gadget: composite: add iSerialNumber to usb_composite_driver

Add iSerialNumber to usb_composite_driver to allow setting a default value.
This is useful when the module is compiled-in. Then the composite_bind
is executed at kernel boot and string id for iSerialNumber can be overridden
even if there is no iSerialNumber kernel commandline parameter.
If the string id is not overridden, then get_string will never attempt to
look for the alternative string contents using cdev->serial_override.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>