V4L/DVB sh_mobile_ceu: don't check platform_get_irq's return value against zero

platform_get_irq returns -ENXIO on failure, so !irq was probably
always true.  Better use (int)irq <= 0.  Note that a return value of
zero is still handled as error even though this could mean irq0.

This is a followup to 305b3228 that
changed the return value of platform_get_irq from 0 to -ENXIO on error.

Cc: David Vrabel <dvrabel@arcom.com>
Cc: Greg Kroah-Hartman <gregkh@suse.de>
Cc: Mauro Carvalho Chehab <mchehab@infradead.org>
Cc: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Cc: Magnus Damm <damm@igel.co.jp>
Cc: Kuninori Morimoto <morimoto.kuninori@renesas.com>
Cc: Paul Mundt <lethal@linux-sh.org>
Cc: linux-media@vger.kernel.org
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>