codecs: wcd_spi: do not use on-stack memory for any transfers

Currently, for some small (1/4/8 bytes) transfers, the driver uses
on-stack memory to perform the transfer. This is not safe as the
underlying master/dma driver could try to map/dma this memory.
Change makes sure to never use on-stack memory for any transfers.

Change-Id: I6b157e26ea7ca9f865004a87153c658756f6c368
Signed-off-by: Bhalchandra Gajare <gajare@codeaurora.org>
Signed-off-by: Xiaoyu Ye <benyxy@codeaurora.org>