mfd: Use i2c_client as an argument on MAX8998 i2c routines

The MAX8998 chip have regulator and rtc features. The i2c slave address
of regulator and rtc is different, so needs each i2c client on i2c
operation functions.

Also, this patch exports i2c operation functions instead of callback to
make easy to read.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>