spi: spi_qsd: Improve latencies for small transfers

For smaller transfers when DMA mode isn't used improve the transfer
latencies by being more effecient when moving data to the hardware
FIFO especially when using bits_per_word that is less than 32. This
will reduce the number of interrupts being generated in such transfers
helping in overall transaction latency.

Change-Id: I6ddd933d4a43a4d24d7b5310a678a19411a14f2c
Signed-off-by: Girish Mahadevan <girishm@codeaurora.org>