USB: serial: cast sizeof() to int when comparing with error code

Negative error code will be larger than sizeof().

Note that none of these bugs prevent errors from being detected, even if
the ir-usb one would cause a less precise debug message to printed.

Signed-off-by: Chengguang Xu <cgxu519@gmx.com>
[ johan: add comment about implications ]
Signed-off-by: Johan Hovold <johan@kernel.org>