mtd: dataflash: Remove use of tx_dma

We are trying to remove the legacy tx_dma and rx_dma fields from the
spi_transfer structure. Currently dataflash uses tx_dma but only to make
sure that it's set to 0 so we can remove this use by replacing with a
zero initialisation of the entire spi_transfer struct.

Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>