usb: linux/usb/serial.h: drop duplicated word in comment

Drop the doubled word "set" in a comment.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-usb@vger.kernel.org
Link: https://lore.kernel.org/r/20200715045701.22949-4-rdunlap@infradead.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>