mfd: twl-core: Move twl_i2c_read/write_u8 to header as inline function

twl_i2c_read/write_u8 become as a simple wrapper over the twl_i2c_read/write.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>