spi: spi-geni-qcom : Don't configure SPI WORD LEN for SPI Slave

Don't configure SPI word length for SPI Slave as it's an optional
property. Hardware will take care of SPI word length field when the
QUPv3 Serial Engine is configured as SPI Slave(SPI_SLAVE_EN is set).

Change-Id: I714f455589be75d19d8f32e9f04e8cdc518a0430
Signed-off-by: Ashish Kori <akori@codeaurora.org>