ASoC: fsl_ssi: Change irq type to 'int'

Since commit 2ffa5310 ("ASoC: fsl_ssi: Fix module unbound") the irq
number is retrieved via platform_get_irq(), which may fail and return a negative
number, so adapt its type to 'int'.

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