usb: gadget serial: don't shift bcd version left by 16

Fengguang Wu reported:
|drivers/usb/gadget/serial.c:89:22: sparse: cast truncates bits from
|constant value (24000000 becomes 0)

I obviously let the version number shift away. Since the version is a
16bit number it can be applied as it.

Cc: fengguang.wu@intel.com
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>