mfd: Remove checking on max8925 regulator[0]

Since regulator[0] is always checking in mfd driver, it results in
registration failure without regulator[0].

Signed-off-by: Haojian Zhuang <haojian.zhuang@marvell.com>
Reviewed-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>