mfd: Fix mc13xxx SPI regmap

This fix the SPI regmap configuration, the wrong write flag was used.
Also, bits_per_word should not be set as the regmap spi implementation
uses a 8bits transfert granularity.

Signed-off-by: Philippe Rétornaz <philippe.retornaz@epfl.ch>
Tested-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>