serial:ifx6x60:Keep word size accordance with SPI controller

As protocol driver, IFX SPI driver initiate to setup SPI master with default
SPI word size as 16 bit/word, however, SPI master may not adopt this default
value due to SPI controller's capability, it might choose an available value by
itself and set it to spi_device.bits_per_word. In order to keep align with
Controller, IFX driver should make use of this value during SPI transfer,
but the default one.

Signed-off-by: Chen Jun <jun.d.chen@intel.com>
Signed-off-by: channing <chao.bi@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>