When building with clang, the compiler compains with
drivers/spi/spi-synquacer.c:212:11: warning: variable 'bus_width' is used uninitialized whenever 'if' condition is false [-Wsometimes-uninitialized]
else if (priv->mode & SPI_TX_OCTAL)
^~~~~~~~~~~~~~~~~~~~~~~~~
drivers/spi/spi-synquacer.c:276:11: note: uninitialized use occurs here
val |= ((bus_width >> 1) << BUS_WIDTH);
^~~~~~~~~
drivers/spi/spi-synquacer.c:212:7: note: remove the 'if' if its condition is always true
else if (priv->mode & SPI_TX_OCTAL)
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/spi/spi-synquacer.c:189:25: note: initialize the variable 'bus_width' to silence this warning
So initialize bus_width to 1 and add a warning if none of the configured
modes matches