drivers: spi: esp32: fix exception in mode 3 & soft-ctrld CS

Fixes division by zero exception when SPI is configured to operate in mode3
whit CS controlled by software.

Such exception occurred because data->dfs is listed as the denominator in
a division inside the spi_esp32_transfer() function which is called from
spi_esp32_configure() (before assigning a value to data->dfs) inside
transceive() in the condition where mode 3 is chosen as the SPI operating
mode and the chip select is configured to be software-controlled.

Signed-off-by: Marcio Ribeiro <marcio.ribeiro@espressif.com>