usb: truncate the most significant part of the serial number

When the Hardware Information Driver (HWINFO) provides a device ID
longer than the configured USB serial number, truncate the most
significant part instead of the least significant part. The lower part
is usually having more entropy, e.g. on STM32.

Fixes #34550

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>