From ARM system, system_serial is identical serial number. So, this
patch fixes to use arm system_serial as serial of lagacy slp usb
gadget instead of fixed serial if there is no dmi serial value.
Change-Id: Ie28c6c3d2afc54fc1ea54d3eb67100d3ed27165d Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>