ASoC: fsl_ssi: Fix irq error check

Commit 2ffa5310 ("ASoC: fsl_ssi: Fix module unbound") changed the way to
retrieve the irq number from irq_of_parse_and_map() to platform_get_irq(), but
missed to updated the irq error check accordingly.

We should test for negative irq number and propagate it in the case of error.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Mark Brown <broonie@kernel.org>