USB: serial: use IS_ENABLED() instead of checking for FOO || FOO_MODULE

The IS_ENABLED() macro checks if a Kconfig symbol has been enabled either
built-in or as a module, use that macro instead of open coding the same.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Johan Hovold <johan@kernel.org>