ASoC: nuc900: Fix platform_get_irq() error checking some more

The error handling doesn't work here because "nuc900_audio->irq_num" is
unsigned.  Also we should be checking for < 0 and not <= 0 but I believe
that's harmless.  The platform_get_irq() comments don't talk about the
return values...

Fixes: fa8cc381 ("ASoC: nuc900: Fix platform_get_irq's error checking")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mark Brown <broonie@kernel.org>