drivers: usb: device: resolve USB build error for STM32N6

- Removing the unused declaration of 'ret'
  in the usb_dc_stm32_init function for STM32N6.
- Ensuring the 'ret' variable is used correctly
  in the pinctrl_apply_state function call.

Signed-off-by: IBEN EL HADJ MESSAOUD Marwa <marwa.ibenelhadjmessaoud-ext@st.com>