mfd: palmas: Check if irq is valid

Check if irq value obtained is valid. If it is not valid
then skip the irq request step and go ahead with the probe.

Signed-off-by: J Keerthy <j-keerthy@ti.com>
Reviewed-by: Mark Brown <broonie@linaro.org>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>