uboot-imx

usb: gadget: g_dnl: only set iSerialNumber if we have a serial#

We don't want to claim that we support a serial number string and
later return nothing. Because of that, if g_dnl_serial is an empty
string, let's skip setting iSerialNumber to a valid number.

Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>