DW SPI: fix transmit only mode

In current implementation we get -ETIMEDOUT error when we try to use
transmit only mode (SPI_TMOD_TO)
This happens because in transmit only mode input FIFO never gets any data
which breaks our logic in dw_reader(): we are waiting until RX data will be
ready in dw_reader, but this newer happens, so we return with error.

Fix that by using SPI_TMOD_TR instead of SPI_TMOD_TO which allows to use
RX FIFO.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>