spi: ep93xx: update kerneldoc comments for ep93xx_spi

spi: ep93xx: update kerneldoc comments for ep93xx_spi

Two fields got removed but are still documented:

drivers/spi/spi-ep93xx.c:98: warning: Excess struct member 'dma_rx_data' description in 'ep93xx_spi'
drivers/spi/spi-ep93xx.c:98: warning: Excess struct member 'dma_tx_data' description in 'ep93xx_spi'

Fixes: 3cfe73256905 ("spi: ep93xx: add DT support for Cirrus EP93xx")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>