usb: gadget: u_serial: fix typo which cause build warning

fix typo error introduced by commit ea0e6276 (usb: gadget: add
multiple definition guards) which causes the following build warning:

  warning: "pr_vdebug" redefined

Signed-off-by: Bo Shen <voice.shen@atmel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>