USB: serial: fix boolinit.cocci warnings

Bool initializations should use true and false. Bool tests don't need
comparisons.

Also, use IS_ENABLED instead of ifdef.

Generated by: scripts/coccinelle/misc/boolinit.cocci

Signed-off-by: Mathieu OTHACEHE <m.othacehe@gmail.com>
Signed-off-by: Johan Hovold <johan@kernel.org>