USB: serial: fix repeated word "the" in comments

There is a repeated word "the" in two comments that should be replaced
or removed.

Signed-off-by: Jiang Jian <jiangjian@cdjrlc.com>
[ johan: replace one "the" with "that", merge the two cleanups, amend
         commit message ]
Signed-off-by: Johan Hovold <johan@kernel.org>