mfd: Use to_i2c_client() instead of open-coding it

drivers/mfd/88pm80x.c

drivers/mfd/88pm860x-core.c

drivers/mfd/max14577.c

drivers/mfd/max77686.c

drivers/mfd/max77693.c

drivers/mfd/max77843.c

drivers/mfd/max8925-i2c.c

drivers/mfd/max8997.c

drivers/mfd/max8998.c

drivers/mfd/sec-core.c