spi: synquacer: remove SPI_TX_BYTE handling

Current code expects that SPI_TX_BYTE is single bit mode
but it is wrong. It indicates byte program mode,
not single bit mode.

If SPI_TX_DUAL, SPI_TX_QUAD and SPI_TX_OCTAL bits are not set,
the default transfer bus width is single bit.

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>