ASoC: mxs-saif: Use a signed integer for error value

saif->id and saif->master_id are unsigned, so they can not be negative.

Fix the following warning when building with W=1 option:

sound/soc/mxs/mxs-saif.c: In function 'mxs_saif_probe':
sound/soc/mxs/mxs-saif.c:676:2: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]
sound/soc/mxs/mxs-saif.c:688:3: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]
sound/soc/mxs/mxs-saif.c:692:2: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]

Use a signed variable 'ret' to handle the error values.

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