usb: fix CDC ACM breakage with CONFIG_USB_DEVICE_SOF

Commit e4c447aa ("usb: add SoF event") added support for calling the
status callback with the USB_DC_SOF value for each SoF when
CONFIG_USB_DEVICE_SOF is enabled. The CDC ACM driver saves the latest
received status and compares it USB_DC_CONFIGURED to decide if it can
send data to the host. This therefore doesn't work any more when the
status callback is called regulary with USB_DC_SOF.

Fix that by ignoring USB_DC_SOF when saving the latest received status.

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