usb: gadget: Call g_dnl_bind_fixup() before testing g_dnl_serial length

usb: gadget: Call g_dnl_bind_fixup() before testing g_dnl_serial length

After the commit SHA1: 842778a091 - the serial number descriptor is only
visible when we have non zero length of g_dnl_serial.

However, on some platforms (e.g. Siemens) the serial number is set at
g_dnl_bind_fixup(), so with the current code we will always omit the
serial (since it is not set).

This commit moves the g_dnl_bind_fixup() call before the g_dnl_serial
length test.

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Acked-by: Heiko Schocher <hs@denx.de>
Tested-by: Heiko Schocher <hs@denx.de>