drivers: spi: esp32xx: Fix word size limiting transaction

Word size setting limited any SPI transaction to the frame
size. In addition to making the SPI inefficient this
broke drivers that set the word size. It appears that most
drivers use a one byte (8) size for this setting.
This change respects what I think is the intended use of
the word size setting. That is to set the length of
each element in a tx/rx buffer struct.

Signed-off-by: Dean Sellers <dsellers@evos.com.au>