CODEOWNERS: Update path to usb_dfu.c

This is a follow-up to commit 1c89837e.

Update the path to dfu_usb.c after the file was moved, to prevent
compliance checks from failing because of non-existing file listed
in CODEOWNERS.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>