drivers: flash: spi_nor: optimize SPI buffer usage

This driver was providing SPI buffers for both TX and RX on the data
payload portion of read transfers, even though the TX buffer is not
meaningful in these cases. As well as being less efficient, this also
caused likely uninitialized data to be transferred to the device, which
is possibly problematic.

Update to not include the TX buffer for the read data payload SPI
transfer, so that the SPI driver can generate dummy TX data internally.

Signed-off-by: Robert Hancock <robert.hancock@calian.com>