mtd: spi_dataflash: Use spi read then write

Now, we have spi_write_then_read routine that would handle
spi_xfer handling based on the tx_buf and rx_buf parameters.

So, replace individual flash read/write/cmd transfer call
with spi_write_then_read.

Cc: Egnite GmbH <info@egnite.de>
Cc: Daniel Gorsulowski <daniel.gorsulowski@esd.eu>
Cc: Ilko Iliev <iliev@ronetix.at>
Cc: Marek Vasut <marex@denx.de>
Cc: Mateusz Kulikowski <mateusz.kulikowski@gmail.com>
Cc: Alison Wang <alison.wang@nxp.com>
Tested-by: Adam Ford <aford173@gmail.com> #da850-evm
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>