usb: gadget: g_dnl: Sync internal SN variable with env

Since commit 842778a0 ("usb: gadget: g_dnl: only set iSerialNumber
if we have a serial#") "fastboot devices" stopped to show correct device
serial number for TI boards, showing this line instead:

    ????????????	fastboot

This is because serial# env variable could be set after g_dnl gadget was
initialized (e.g. by using env_set() in the board file).

To fix this, let's update internal serial number variable (g_dnl_serial)
when "serial#" env var is changed.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Cc: Felipe Balbi <felipe.balbi@linux.intel.com>
Tested-by: Heiko Schocher <hs@denx.de>
Acked-by: Łukasz Majewski <lukma@denx.de>
Reviewed-by: Marek Vasut <marex@denx.de>