spi: ti-qspi: Simplify qspi_write_msg and qspi_read_msg implementation

Make the unit of wlen to be byte, and simplify the code to avoid duplicate
code for different wlen cases.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Sourav Poddar <sourav.poddar@ti.com>
Signed-off-by: Mark Brown <broonie@linaro.org>