drivers: spi: fix the update of spi_context tx & rx length

The unit of spi_context's tx_len and rx_len is byte instead of frame.
Thus, in spi_context_update_tx(), the value of ctx->tx_len should be
reduced by (len * dfs).
Also to fix the update of ctx->tx_len in spi_context_update_rx().

Signed-off-by: cyliang tw <cyliang@nuvoton.com>